this might help
when you add a transition in FCP, the transition doesn't cut into the existing media that you see on the timeline, rather it goes 15 frames before your in point and starts the transition there, it also does the same for the preceding clip, going 15 frames AFTER the outpoint. If you have your in points and out points set so that they start right on frame one of the media you cut it from, then when you add the transition it doesn't have those 15 frames to go back on and start the transition, because your in point is already on frame 1. you need to make sure that you have atleast a 15 frame buffer on either side of your media, so that when you add the transition it has those frames to start the transition. I know this sounds a little confusing, but hopefully it should help.
- Ryan