rip the cotton off a Q tip to use a tool, and make sure that you didn't end up with a piece of blow primer in the key channel.
If the B/C was assembled correctly (extractor to the ejection port side of the receiver), then take a look inside the upper receiver to make sure that you don't have debries or a broken parts that is not allowing the Forward assist pawn to retract all the way back into is channel.
Short that that, remove the charging handle and see if you can get the bolt to lock home without it. If you can, then the end of your charging handle in the receiver is screwed up, and blocking the carrier from going all the way forward.
FYI, the charging handle pulls the carrier back via the flat on top of the carrier in front of the key (not from the end of the key). Hence could be that you did not install the charging handle correctly if it in spec, its hanging low in the front, and why the front of the carrier can not get past it.