Abnormaly tight reassembly of a KA reviever. why?

[airsoftjunky]

So I've had my King Arms M4 apart before, and re assembly wasn't any more difficult than usual. Yesterday I tried to take it apart again to put a new hop up and hop unit in, and I nearly had to hammer it apart. Also when reassembling it, same deal. Had to use extreme force. With out the hop up and barrel in, no problem. With it in, even tried the stock one, I had to bump the but of the gun on the floor hard to get the upper and lower receiver back together. I've never seen them this tight before. Any ideas why?