Some time ago I had a problem with a UIWebView myself. The video was not showing all the time (sometimes it did sometimes it didn't).
This is not exactly your problem, but keep reading. ;) What I discover was that the constraints are very important for the webview. If you did not set the constraints please set them and double check them if they are set probably.
In my case the video was retrying to be played in the view. This was causing a memory leak in the program. So the constraints can be a factor in the memory leak.
Hope this helps!