What could prevent me from buying a gun?

I went to go buy a gun from a sporting goods store and they did a background check. They called me back a couple days later and told me they couldn't sell it to me. They said they didn't know why I wasn't allowed to buy a gun but I only have a felony charge as a juvenile (which was expunged and adjudicated) and no warrants or even traffic tickets. The only thing I could think of is that my brother has the same address as me and he is a convicted felon. Now he doesn't actually live with me but his state ID says he does, could that be why?