I can't really answer why other than to say it happens to me alot and I ususally find out it's some little thing I am overlooking that it doesn't respond right. I hate that because it really frustrates me.
Can you give us as much specifics as possible? You know, like you were doing a challenge description. Kind of walk us through it step by step. What are your pallette settings, what brushes are you using any other pertinent info and maybe we can all fool around with it and try to reproduce what you are doing and why it's greying out the filters you want. Unless someone can tell right away for you. That would be great too. I however, usually need things spelled out first.
DJ