If you try to post sync a video that you have made with your phone, an issue can occur where the video only has a couple of frames. This happens because some H264 encoded videos use B-Frames and multiple backwards references to B and Key Frames on intermediate frames. This is good for video playback, but it does prevent "frame accurate seek", which is what Dewesoft uses to position itself on certain frame.
To sync a video like this you need to convert the video with FFMPEG using this command:
ffmpeg -i BframeVideo.mp4 -vcodec h264 -intra -x264-params bframes=0:keyint=4:min-keyint=4:scenecut=-1 -max_muxing_queue_size 9999 NoBframeVideo.mp4
This will properly encode H264 video to MP4 format which can then be used by Dewesoft.