SM465 problems

I've just about got the new Novak mainshaft installed in my SM465, but I've run into a problem. I'm trying to put the top cover back on, but the shift fork closest to the front of the trans is about an inch too far forward. The problem is that the shifting collar in the trans is too far back, but I can't get it to slide forwards. It's right up against the gear behind it, and it won't move at all. Any ideas what could cause this?

Edit: I just looked it up, it's the 3rd to 4th gear slider that's stuck, and it's currently right up against 3rd gear.