; Wont listen to me

I’ve got a movie clip that i wanna trigger from a button. Ive done the usual on release; but it just wont play. It’s not the button, coz ive tried it using a different mc as the target. it’s not the name coz I used the same name. The only thing I can see that’s different is that the target is in a mask, and is also sitting on a keyframe as part of a motion tween. This doesn’t matter right coz I’m sure ive done it in these circumstances many times before? I must have… Thanks in advance.