72 hours by federal law (and I'm not sure exactly how they count it), makes it legal for a "default proceed" - but that's only federal law, you can have state laws, you can have individual store policies. Some stores might not ever transfer a gun on a default proceed, some might always, some might sometimes, some might after a month, some might to you and not some other person or vice versa. Usually if you get a delay, a good business is going to ask "OK so do you want to cancel it or wait?".
You also didn't mention the state - no idea of what is out there but states can have their own laws about delays, and can also be the one doing the checks.
In NH - any sale only involving rifles, or "other" firearms (which means AR lowers not marked as pistols basically), goes through normal NICS, while any sale involving a pistol (or a pistol and other guns) goes through the state police... My point is, to be accurate, it's not necessarily the feds doing the check, depending on your specific state/laws.
My 3rd gun purchase this year produced a 6 day delay, the two before it did not, the one after it did not. Such crap is par for the course.