Hello? You have the right to
find out why you were denied. You don't need to guess.
If the dealer can't tell you, go to this site. It'll tell you where to write or email (and what info to send) to find out why you were denied, and how to appeal if your denial was improper.
Federal Bureau of Investigation - NICS E-Check
If it turns out to be the pot charges, you can usually get something that minor expunged from your record. It may take you a while, but it would definitely beat illegally possessing firearms, and having someone else break the law by being a straw purchaser for you. The feds are not amused by that sort of shenanigans.