Never had this happen. How about a close up picture of the setup?
The chainring nut is wider than the bolt. Is this still smaller than the ring or is the bolt not extending through to the ring hole? Have you tried another set of nuts? They are cheap enough that you might want to do that. Also, are the chainring bolts/nuts single speed specific? If not, I've seen the nuts bottom out and the setup will seem like it's tight but actually has a little play in it.
Running fixed off-road you really shouldn't be putting enough back pressure on the setup that it slips easily unless something else is going on. Without seeing in person I'm guessing the bolts are a likely problem.