I've had this problem too. Even with everything in proper working order and torqued down nicely, I still sometimes pull a rear wheel forward in the dropouts. I'm a big heavy guy and these are fixed gear bikes so the top of the chain is just under a lot of tension when I'm climbing a steep hill.

My solution was a Surly Tuggnut. They only market them for track forks but I've been able to install them on a few frames with forward-facing rear dropouts after a little finagling.