LPCxpresso instruction stepping

I would have posted this over on the LPXxpresso forum but my password isn't working over there and it seems no matter what I enter to answer the "what part number are you interested" random question it doesn't like it.

Anyway, I've just been debugging some code and my instruction stepping is now greyed out. I have the Disassembly view open and in focus but F5 still steps by source code, not assembly. I had that working before, don't know when it quit working.