Good question. I believe that all doms must experience this from time to time. Espeecially when you are living it. The best thing is to be honest. It does no one any good to play if you are not into it. The emotional demand is to high to go into a scene empty.
If I am not into it on a particular night I will say so. Then we can do someting else that is not so demanding.

This ability to communicate is what tells you you are with a partner who is on the same page as you. And that is, afterall, a very important thing.