Unable to add more transitions

Hi all. In Windows Moviemaker, I can drag and drop a
load of .WMV or .AVI files onto the storyboard, but when
I start adding video transitions, all goes well for a few
but then all of a sudden, I won't be allowed to add
more. When I try to drop them into the correct spot on
the storyboard, I get a black barred circle.

I am using Win XP Pro with SP-1 and all relevent updates,

Andy