Wrote about this in a review on the AppStore and the developers sent me here. Basically when I import a video to edit it will show that the video is there but it won't play the video on the preview screen. When this happens the video will sometimes show a key frame but that's it. I've deleted and reinstalled the app. I've deleted and tried to restart my video. I've checked to make sure the video is playing correctly from my camera roll. I've shut off and restarted my phone. Nothing seems to work. It only does this with some videos and it doesn't seem like the length of the video effects it because this has happened to both hour long videos and 30 second videos.
I'm editing through an IPhone X. Most of the videos are downloaded offline. When I've used them on other applications they've worked (if that gives any insight). I think the video format is MP4 but I don't know how to check that on IPhone and they're typically 720p or 1080p. Sorry if I'm answering the questions wrong, I'm not much of a computer buff. Thank you for responding!