Cause : Usually this happens when the video or audio player you used to play the file doesn’t have necessary Video codec support.

Solution : You can use the VLC or MX Player to play the video. These players support almost all video codecs.

Choosing the right codec is very important in case of playing output video file in your preferred player properly. Different devices support different codecs.

For example, h264 video codec is best when you want a video tp play on the android media players.

To resolve quality loss issue, use recommneded codec. There's an options to choose video quality under compress screen. Higher quality means larger output file size.

Increasing resolution won't help in this case because there's no technology yet which can improve original video quality.

If the input file is already compressed then it may not possible to compress it more.