So, I have a 9mm AR kit that I assembled.  I am by no means new at building or servicing ARs.  Went to finally assemble upper to lower and the fit was quite tight.  Takes a lot of work to get receivers together so the takedown pins can go in.  Charging handle and BCG have no issues in the upper disassembled.  The spring and buffer also move well in the tube.  However, when I put them together, I am unable to even pull back on handle.  It's obviously stuck on something......I guess.  I checked the buffer retainer and made sure it didn't interfere with the bolt.  Everything looked fine........kind of lost now.  I will field strip tomm and see if anything stands out.  Any advice or suggestions?

Is the upper and the lower receiver from the same man'f?  Who(m)???

I have seen the ejector bind on bolt. Also my last one had the bolt hold open drag on bolt. I would check these. Sounds like tolerance stacking. Is it a colt or Glock build?

Good chance it's the ejector, it has to fit into a cut path in the bolt and sometimes they don't line up perfect and bear against each other.  You can watch as you slowly close the upper, does the tightness start when the ejector finger starts sliding along the bolt?

You might need to slightly bend the ejector away from the bolt surface it rides in to loosen the fit.

Rear end of the upper is sitting slightly higher than it should be? I’ve had this in a few builds: One I was able to get running by removing the anodizing on the real lug of the upper receiver. After that the pins pushed in easier and the bolt could be drawn fully to the rear. The other wasn’t close to lining it so I just paired it with a different lower and problem solved. I didn’t bother tying to figure out if it was the upper of the lower that was out of spec.

This ^^^

I have to agree. It sure sounds like the ejector, or possibly the whole mag adaptor if being used, is sitting too high. One should not have to mess with the receiver lugs to get the upper to fit onto the lower.