Only works with some videos – Convert - GIF & Live Wallpaper Review

I downloaded and opened the app. It asked for permission to access photos and I chose allow access to all photos. But when I chose a video to convert, it showed a pop-up box saying please wait and the disappeared within a split second. Keeps going on like this so I can’t do anything at all. It has only worked (I was able to trim the video and converted it) one time for me after so many attempts and then not working again. Edited: it looks like it only works with some videos, and it doesn’t seem like the length of the video matters. So I don’t really know what the problem is.
Review by uber buggy on Convert - GIF & Live Wallpaper.