When you spend a great deal of time writing for a living, there seems to be this point where you don't so much hit a wall, as the wall just flat out falls on top of you, or your muse decides she's bored and roams off with a bottle of scotch and a stack of books and locks herself in the attic. Writing for fun is one thing because often your mind is able to latch onto some sense of direction and keep fairly open to more abstract ideas.
When you're facing a deadline or a confined “theme,” it can get insanely difficult to find that spark to keep rolling. I wish I could go into just how insanely common this is and how frustrating it is. It's something I hear about almost constantly from other writers, from virtually every form of media.
There is something about it I don't think anyone really expected, more when you're allowed to basically write what you want (granted it's within a certain topic, which is alright because you enjoy it). The problem is that eventually you run out of things. You think “tomorrow or next week won't be so bad.” Then next week is just as bad.
So how and the hell do you work around that? It is your job after all, it's not like you can shelf it and take up a new hobby. To be honest, I've learned that the best things sometimes really do pop up by accident when you're not thinking about it.
You may be rambling with a friend about some totally silly topic and right there you find your topic. This seems to be a good guide for many things, just let go now and then, let things come to you. You'll go mad trying to seek them out yourself.