NOTE: This is a generalisation and there are workarounds, but it does apply in most cases. I'm not going to go into the workarounds here because they're unreliable at best.
To check if a video will work with Showtime, load it in MediaInfo and look at what it says in the first pair of brackets after AVC.
If it's 4.2 or lower then it'll work fine:
If, however, it's 5.0 or higher then it won't work: