I am building a prototype for a mobile web site/app and was successfully rendering out an animation happening in the header area as both animated GIF and h264…when I added another animation of a layer sliding out the left side of the screen and re-rendered the animation, this added animation is not being rendered out. The layer stays in its place and isn’t moving, like it should.
Where to begin trouble-shooting something like this?
for yourself: have a look for errors in the console
for others to help: add a sampledocument
Haha, touché! 
I would post the document, but a) it is client project which I shouldn’t post, and b) there are/were so many strange things happening that I just put my head down and kept trying until I got the results I needed, often through very unconventional methods.
For example, one text element appears in a scene even though it is at 0% opacity, which I had to solve by covering it up with another element (which worked in my specific).
In any case, there still appear to be a few bugs to squash in Hype 4 Pro, and in the meantime, creative DIY patches will be necessary, I guess. Still an amazing piece of software. 
I think I have had in the past some elements show when set to 0 opacity because there was a conflicting keyframe action on one timeline at the start.
Hm, interesting.
I do have two timelines, but am using only one…I shall try setting up a corresponding keyframe for that element in the unused timeline, also set to 0% opacity, to see whether that fixes it.