PE8 won't allow transition to be applied

Question

I've seen this before from time to time, where I try to drag a transition to the timeline between two clips and it simply disappears when I let go of the mouse... in what cases does PE8, by design, prevent a transition from being applied between clips?

Answer 1

Steve has probably nailed this problem, but there is another consideration, when applying Transitions. For nearly all Transitions, one will need Handles on the end(s) of the Clips, where it is to be applied. These are extra Frames, before, or beyond the set In & Out Points in the Clips. This will explain Handles a bit more:
If one does not have adequate Handles, two things can happen: Frames from what the user thinks are from the next scene can appear, or PrE will warn you and duplicate the last Frame of the Clip to provide Handles. This can cause the scene to "judder," in an unintended manner.
Handles were easier to grasp, as a concept, when one could only do A-B Roll editing, where they could see the Clips overlapping to allow for Transitions, as the Clips were always on separate Video Tracks. With NLE's like PrE, editing is most often done on a single Video Track, and one does not see the Handles. This will give you an idea of what things look like in A-B Roll editing:
Hope that this helps, and saves the problem, when confronting Clips without adequate Handles.
Good luck,
Hunt