Big pet peeve of mine.

Not showing up is rare, but showing up late or very late, is common. On my last trip out of SFO they were already boarding zone 3 before the cart showed up. Part of the issue is the duty free store (or kiosk) is staffed with one person and they have to close their kiosk and head to the flight, which they can't do if they're serving other customers. I remember last year at LAS running back to the kiosk demanding my stuff telling them my flight was boarding. The flustered staffer had to close her kiosk while several puzzled customers continued to try to ask her dozens of detailed questions about scotch, none of which she could answer.

Adding to the problem is the fact that many US airports don't have "International Terminals" so you can't just take your purchases with you. As there are both domestic and int'l flights departing from a given terminal you're dependent on a clerk pushing up a cart.

It's pretty much to the point where I don't buy duty free departing the USA any more - Not worth the stress.