One of the most important facets of my writing process is that I'm an incubator. I don't usually tackle ideas right away: instead, I put them on the backburner of my brain to simmer, develop, resolve problems and fuse with other thoughts. This process occurs under the surface - occasionally, something will occur in the front of my brain and I'll drop it in the pot, as it were, but most of the time, it's neither active nor conscious.
I think this is part of the reason why I tend to do extensive world and character building before I start writing: it gives my mind time to turn over possibilities and permutations and actively brings them to the fore. It also explains why I often have trouble simply diving into a story as soon as the idea strikes me - I can't develop it on the fly, nor can I rush the process. Oddly (or not?), this also applies to me and arranging music on the harp.
So I've learned to listen to my writer brain and determine when reluctance is laziness or procrastination, and when it's my subconscious telling me the idea hasn't had enough time to brew. I've learned to be kind to myself when I fall down on my writing goals - within reason - because my brain never stops. It's all there, waiting for me.