ORIGINAL: GMMAT
I know mine "locks" in the fully upright position. You can also do this manually to check. It'll lock with the thumb tab, also.
I always lock mine up before shooting, but if you read the manual you will see it says the rest has to go to at least 90 degrees to work properly. This is how I found my problem. I'm pretty confident this should fix the problem.
Let us know what you find out.