Final cut express 4, audio effect cross fade won't work

Hey you guys,

Question, i've just started working with FCE4 (used to work with Premiere Pro) and now i've stumbled on to something.

I added a couple of clips to my sequence and now i want to add a audio effect to all of these clips. The cross dissolve effect works on a couple of clips, but on a couple other it won't attach to..

Does anyone know what this could be?? Thanks ya'll