Personally, I'd never say no to a novel idea to work on a picture book challenge/idea, but I'm a noveler (is that even a word?). To answer your original question, and it may sound easier said, but I make myself. If I'm sitting in front of a manscript and I can't get focused, I get rid of all backgound noise, make sure my needs are met (usually something to drink), and I close my eyes, breathe, tell myself to get it together, and do so. It's enough to let me jump from, say, a mg fantasy to a ya contemp.