I can't add transitions at all to any clip in the timeline.

No matter how I try to add the transition, placing the play head where I want it and double clicking the transition in the browser, selecting the clip and double clicking the transition, dragging & dropping the transition I get the error:

There is not enough extra media beyond clip edges to create the transition.

The clips I'm trying to add the transition to have about 15 seconds of footage extending beyond the cut in either direction. It won't let me add transitions to any other clips anywhere else in the timeline or inside any other compound clips either.