Split Screen Video with Transition Effect only in one Video

The answer that resolved it for me was that it is certain transitions. I was initially trying the 'fade to white' and 'fade to black', but it's just about any other and works perfectly.  Like I said with a few exceptions.

Thanks for the support

In order for a transition to work you need "handles" on both sides of the media.

If you don't have handles, there's no media to make the transition.

You will need to trim from the ends to create the handles.