Sometimes it will actually make you wait for it to be delivered to you garage, like when I ordered my tank, it took almost 10 min (IRL) to get it delivered to my hanger.

Try switching to another character and switching back and looking to see if its there yet.

It said it was delivered but to what hanger/garage?
I bought it on Michael

im having the same problem, I have ordered the tank on Micheal & Franklin & there not anywhere, I've checked the safe house, Garage, Hanger at airport, Helipad & dock & its no where to be found, I've also went the fort zucuta (or what ever its called) & they tried to kill me as soon as i entered there

if its a tank it goes to a hangar, if its a car it goes to the garage you bought, if its a helicopter it goes to helipads, you have to wait a little before youu receive it, like go away from the place for a short time until it says delivered