Before proceeding, please make sure you're working with the latest version of Premiere Composer.
Important! Are you on a Mac with M1 processor? If yes, you might be experiencing a different issue. Checkout this guide instead.
1. Select all of the transition clips
2. Right click on the Premiere Composer's "Menu" button to open a hidden dropdown. Important! right click is the other mouse button, this is a hidden menu.
3. Press "Fix selected clips"
Like on the following gif:
Seeing a black screen instead?
It's possible that there was a problem and the plugin set your footage layer as "adjustment" by accident. Make sure your video clips under the transition are not set as adjustment. Here is how to check:
It this keeps happening. Please try to drag the transition into the timeline instead of using "Add" or double-clicking the item.
If this still doesn't work. It is also possible, if you're using an adjustment layer above your footage that it's "adjustment" setting was unset because of a problem. Try to right click on adjustment layer and click "adjustment", making sure it's on.
Disable GPU rendering
Here is how this can be done in project settings:
If the transitions will start to work, then it means Premiere has some problem with the GPU you're using. Perhaps updating GPU drivers might help.
Please let us know if none of the suggestions helped.