So I recently purchased a complete pistol lower and a complete upper to build an AR pistol. Been about a week my lower receiver shipped out the day after I ordered but my upper has yet to ship. I have never bought a lower and upper. I have only ever purchased already built guns from PSA. Is this a common occurrence for one to ship and not the other? Not sure why they don't just ship both lower and upper at the same time if it was bought at the same time..