Even if the format is correct, the codec mismatch can cause playback issues. I had a situation where a device only supported MJPEG, but my converted files ended up using H.264. The best fix was manually selecting the right codec in a different converter since not all software gives codec customization options. If you need more control over formats and codecs, an alternative converter might help. I've had good results with this https://www.movavi.com/video-converter/ram-converter.html, which lets you choose specific output settings to match device requirements.