When in doubt, write it out.
Sometimes, when you just can't work out what to do next, it's a good idea to have a conversation with yourself. I like to do that in writing, so that I can see what I think, literally. I remind myself what my intentions are/were for the piece, and then work out what is getting in the way of achieving that. Tomorrow, I'll show you an example of how you can work with a piece you are stuck with.
Of course, it doesn't just work for being stuck with a piece; it can help you to formulate ideas for the next piece, particularly if you are working in series. Then, you have a written record of where the series is going; sometimes, it helps to be able to look back at a written record to see how your thinking has developed over a period of time.