I believe the issue is actually with your buffer not allowing quite enough rearward travel. Have a look at the MS build forum other people had the same issue.
I removed the buffer and buffer spring entirely and tried it again. Using the charging handle and moving it slowly I am unable to get the bolt back far enough to lock on the bolt catch. I can stick in my finger and push it further back so the bolt catch can engage. Doing this I've determined its not a buffer issue. It's a the receiver to too short to allow proper movement of the charging handle issue. The receiver itself (total length of upper and lower) is slightly shorter than a standard ar15.