can't insert video transitions between some segments - FCE 4.0.1

I'm only a casual user, so this is likely due to my lack of knowledge, but I can't find the answer. I have series of video/audio segments in my timeline, and I can't insert any of the video transitions between certain clips. If I slide down the timeline to the next break between video segments, it inserts fine. Moving down the timeline, some breaks allow the insertion, some don't. I can't find a pattern which explains this behavior.

If I slide the 2nd segment slightly to the right creating a gap between the segments, I can insert a transition on either side of the gap. Only when the segments are completely contiguous am I unable to insert.

Video generators, such as text, insert anywhere just fine. I have a second sequence in this project in which I had no problems inserting transitions at any location. Any ideas why FCE is unable to insert these transitions?