I have a Colt Match Competition for the late 1990's.  It was originally a 20 inch HBAR with an A2 stock but, due to CA's silly AW laws, I swapped out the upper and put a carbine stock on it and put the Colt parts on another rifle.  Both rifles worked fine.

I'm no longer in CA so I decided to put the Colt parts back onto the Colt receiver.  I removed the carbine stock parts, FCG, pistol grip and safety.  I removed the FCG thinking I could swap for the better trigger in my other rifle but they don't fit so they went back into the Colt.  I put the A2 stock back on, reassembled it all, and that's when I ran into the problem.  The upper and lower are difficult to fit and it needs to be squeezed tightly to get the rear pin in place which is difficult to push in.  The rifle originally had a plastic accu wedge but that got discarded long ago.  With the upper and lower pinned together, the bolt cannot be pulled back.  It is apparently misaligned, probably due to the stock and related parts being installed incorrectly, but I can't figure it out.  All of the parts are the original Colt factory parts. I can operate the bolt with the lower and upper together but not pinned so it isn't misaligned until the very last bit.  Any suggestions of what I should check for?

Buffer retaining pin bound by the receiver extension and binding on the underside of the bolt carrier?

Receiver sear block? Half circle bolt carrier?

It's the original upper and lower which uses a screw type front pin. All of them are the original, factory parts.

I loosened the receiver extension and it seemed to make it easier to close but didn't solve the binding. I tried loosening it a bit more but that didn't help.

It has a receiver sear block.  I don't know what a half circle bolt carrier is.  It looks similar to my other BCG.

I tried a number of different combination.  I swapped the 16 inch upper off my carbine receiver and put it on the Colt lower and it worked fine.  I swapped the 20 inch upper off the Colt and placed it on the carbine lower and it worked fine.  I tried swapping BCG/CH and that made no difference.  It appears to be the combination of the Colt upper and Colt lower that's not aligning correctly.

Thank you, thank you, thank you!
I have the first Colt sear block and was using the wrong carrier.  The original carrier had been switched into a different rifle and once it got put back, it works just fine!

Good to hear!