Happens to experienced designers, too. I finally figured out that I work best under tight constraints. If you said "Kevin, design me a game," I'd flounder uncertainly and never produce anything. If you said "Kevin, design me a game about a one-legged dwarf trying to escape quicksand," I would be all over that in a heartbeat. For me, hyper-focus inspires great ideas -- so if I don't know what to do next, I now tighten constraints and focus in on one thing. I really like the results.