I am the same way with original characters. I know them so well, and their stories and relationships and there are constant developments (like recently Shannon's boyfriend proposed, she freaked out, and they're not together anymore). These developments would make good stories, but when I sit down to write them, it just doesn't happen. LOL I think the problem is now the characters are TOO real to me that writing about them seems weird.

We could do something like we both agree to write a story about our characters (just a short story, or an on going story if we want) like every two weeks or something, and post them to our LJs (with a filter if you don't want everyone to read it). Even if they don't have much to do with the main story (if there is one... my characters just have lives), but just to get us to write on a regular basis.