No, it's really strange. I used two layers to create the button - one for the text and one for the background box. I converted the text and the box to symbols. Then I added a keyframe in the "over" frame for each layer.
Then I created a new layer for the animation, which I intended to trigger when the user passed the mouse over the button. So I'm trying to put a keyframe in the "over" frame of the animation layer, but I simply can't do it. In fact, I've experimented adding other layers and trying to put keyframes in them, and none of these layers will accept keyframes.
I know there must be some very simple reason why this is happening, but I sure can't figure it out.