I've had this happen on both wings that I've built and ARFs. I've put together 2 of the Tower Trainer wings. On both I had to knock the rough corners off the dihedral brace. Also make sure you wiggle the brace front to back as well. If the ribs aren't perfectly aligned you need to coax the brace in.

All that being said, on a wing I'm building now, I had to take almost 1/8" off the dihedral brace to make the halves join correctly.