Both of these manufacturers use the push pin front hinge, so it's not going to be the kind of problem you can sometimes get with a preban Colt.
Wouldn't try to say for sure with out eyeballing the gun, but I suspect that the bottom of the Bushie lug is hitting the shelf under the takedown hole on the RR lower reciever. Rock River uses a higher shelf than the other manufacturers, as a way to pevent the insertion of drop in auto sears. Get something that will leave a mark by just touching; a piece of chalk, an old lipstick, etc, and coat the bottom of that lug on the upper reciever. Then try to close the two pieces a couple of times and see if the marker stuff is coming off on the upper surface of the lower reciever channel. If so, your solution is to file off just a little of that lug. Take it just a couple of strokes at a time, and quit as soon as the upper will go down far enoughto allow the assembly pin to seat.
If the lug is not hitting that channel, you're going to have to take that marker and coat different places on the underside of the upper, to see where it is hitting.