Ok doing an 80% 308 lower, already milled out the FCP and started to assemble the thing. Got it all together and cannot lock the bolt back using the bolt catch. I also cannot get the follower to activate it. You couldn't get it to move using normal finger pressure either locking the bolt back or letting the follower catch it.. and I don't think you could hit it with a hammer and move it. But light pressure on the inside part of the upper paddle and it moves freely.
I inspected the bolt catch retaining pin / screw and it appeared bent, so I ordered a couple more. Made no difference. It will not move except pressing on the top of the paddle from the inside, which you can't reach with the upper installed.
I see or feel nothing in the slot in the receiver, and the catch itself looks and feels fine. The spring and plunger move fine. I'm thinking maybe the catch isn't rotating on the pin correctly? If I back the pin / screw out to the last thread, it seems to move better, but then the pin is barely in there. I've only built three or four AR-10s, of course this one is mine so naturally it's gonna be screwed up....

sometimes, the inside of the upper will interfere with the bolt catch, especially if it's a kit-bash.  you can see this if you put peer into the ejection port with the upper on the lower, no BCG.  easily remedied with some minor dremeling on the bolt catch, gentle radiusing.  don't remove too much, or you'll weaken it.

not sure if this is your issue, though.