It seems after certain action it may happen that a previous transition get's stuck in your current project. When this happens it is not possible to import a transition again. To fix this try the following:
- Locate "Premiere Composer Files" folder inside the project panel and open it
- Look for a sequence named the same way as the transition you're trying to import (it may contain variant name in  brackets)
- If it's there, delete it
Does this happen to you often? Please, let us know. We're trying to find the reason this might happen.