I was cutting out several parts of the .avi video file using LosslessCut 3.29.1
One exported, then later i tried to merge it, but the merged video playback stuck at the end of the first file (i mean playback continued but it it displayed static one frame)

I tried Avidemux which has Append option, it failed to merge too. I also tried ffmpeg command:

Code:
$ ffmpeg -i concat:"a.avi|b.avi" output.avi 
ffmpeg version n4.3.1 Copyright (c) 2000-2020 the FFmpeg developers 
  built with gcc 10.2.0 (GCC) 
  configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-amf --enable-avisynth --enable-cuda-llvm --enable-lto --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libaom --enable-libass --enable-libbl
uray --enable-libdav1d --enable-libdrm --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libjack --enable-libmfx --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enab
le-libopus --enable-libpulse --enable-librav1e --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx26
5 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-nvdec --enable-nvenc --enable-shared --enable-version3 
  libavutil      56. 51.100 / 56. 51.100 
  libavcodec     58. 91.100 / 58. 91.100 
  libavformat    58. 45.100 / 58. 45.100 
  libavdevice    58. 10.100 / 58. 10.100 
  libavfilter     7. 85.100 /  7. 85.100 
  libswscale      5.  7.100 /  5.  7.100 
  libswresample   3.  7.100 /  3.  7.100 
  libpostproc    55.  7.100 / 55.  7.100 
Input #0, avi, from 'concat:a.avi|b.avi': 
  Metadata: 
    encoder         : Lavf58.45.100 
  Duration: 00:00:14.32, start: 0.000000, bitrate: 5075 kb/s 
    Stream #0:0: Video: mpeg4 (Advanced Simple Profile) (XVID / 0x44495658), yuv420p, 704x400 [SAR 101:100 DAR 1111:625], 25 fps, 25 tbr, 25 tbn, 25 tbc 
    Stream #0:1: Audio: ac3 ([0] [0][0] / 0x2000), 44100 Hz, stereo, fltp, 192 kb/s 
Stream mapping: 
  Stream #0:0 -> #0:0 (mpeg4 (native) -> mpeg4 (native)) 
  Stream #0:1 -> #0:1 (ac3 (native) -> mp3 (libmp3lame)) 
Press [q] to stop, [?] for help 
[mpeg4 @ 0x5595056adcc0] Video uses a non-standard and wasteful way to store B-frames ('packed B-frames'). Consider using the mpeg4_unpack_bframes bitstream filter without encoding but stream copy to fix it. 
Output #0, avi, to 'output.avi': 
  Metadata: 
    ISFT            : Lavf58.45.100 
    Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 704x400 [SAR 101:100 DAR 1111:625], q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc 
    Metadata: 
      encoder         : Lavc58.91.100 mpeg4 
    Side data: 
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: N/A 
    Stream #0:1: Audio: mp3 (libmp3lame) (U[0][0][0] / 0x0055), 44100 Hz, stereo, fltp 
    Metadata: 
      encoder         : Lavc58.91.100 libmp3lame 
[mpeg4 @ 0x5595056cf8c0] header damaged2kB time=00:00:14.13 bitrate= 296.8kbits/s speed=7.04x     
[mpeg4 @ 0x559505627cc0] header damaged                                                                                                                                                                                                                                         
Error while decoding stream #0:1: Error number -16976906 occurred                                                                                                                                                                                                               
[mpeg4 @ 0x5595056459c0] header damaged                                                                                                                                                                                                                                                                                                                                                                                                                                             
[mpeg4 @ 0x5595056459c0] header damaged                                                                                                                                                                                                                                         
Error while decoding stream #0:0: Invalid data found when processing input                                                                                                                                                                                                      
[mpeg4 @ 0x5595056adcc0] header damaged                                                                                                                                                                                                                                         
[avi @ 0x55950561f740] Switching to NI mode, due to poor interleaving                                                                                                                                                                                                           
Error while decoding stream #0:0: Invalid data found when processing input 
[mpeg4 @ 0x5595056bea00] header damaged                                                                                                                                                                                                                                         
[ac3 @ 0x5595056496c0] Additional substreams is not implemented. Update your FFmpeg version to the newest one from Git. If the problem still occurs, it means that your file has a feature which has not been implemented.                                                      
[ac3 @ 0x5595056496c0] If you want to help, upload a sample of this file to https://streams.videolan.org/upload/ and contact the ffmpeg-devel mailing list. ([email protected])                                                                                           
Error while decoding stream #0:0: Invalid data found when processing input                                                                                                                                                                                                      
    Last message repeated 3 times                                                                                                                                                                                                                                               
frame=  354 fps=172 q=18.4 Lsize=     791kB time=00:00:20.64 bitrate= 314.1kbits/s speed=  10x     
video:533kB audio:224kB subtitle:0kB other streams:0kB global headers:0kB muxing overhe
What helped was to re-encode video and audio, i used mp4, x264, aac(audio) as output for both cuts and then merge in LosslessCut (it has menu entry for merging video files), worked and playback is OK.
When i reencoded only video, not audio, then the second cut was missing audio in merged video file.