The bolt catch is supposed to catch the face of the bolt itself. (Big surprise! That's why it'c called a bolt catch!)
There's not much to the mechanism. Either the bolt catch can swing up far enough to catch the face of the bolt, or it can't. Assuming the bolt is properly made and not damaged, the only remaining possibilities are:
The machined slot for the bolt catch may not be deep enough (on the side) to allow the catch to swing high enough to catch the bolt face.
If this is the case, it's a defective receiver. Return it. (I'm NOT a fan of DPMS, by the way...)
The bolt catch may be too short. It's improperly made, if so. (Not very likely.)
The bolt catch's slot in the lower may be cut too deep and the pin that retains it may be placed too far down on the lower, which would again be a defective lower.
That's about all it can be.
Try this: Completely separate the upper receiver from the lower. Take the bolt and carrier and put it in its correct place on the lower receiver, pushed back into the buffer tube, holding it firmly in your hand to keep it from being tossed by the recoil buffer spring. Push the bolt catch upward with your thumb while holding the bolt in place, and see if the bolt catch even comes close to holding the bolt.
The bolt catch should be free to swing up to meet the bolt face, and the amount of movement required is about an eighth of an inch upward, at the minimum. On one of my rifles, it'll go up about a quarter of an inch, and on the other it's just a little bit less than that. About 3/16th of an inch.
CJ