Last year I booked a car in San Juan with about 90 minutes notice. They were showing as unavailable when not logged in, but once I logged in (as PC) cars were available.

Got to the location and there were maybe 6-8 groups of people clearly waiting for cars (plus a few more on the bus with me), and a lot with about 2 cars in it. As we pulled up, the bus driver pointed to one of the cars and told me it was mine.

As others have said, don't presume that you're necessarily "behind" the people waiting in front of you - depending on your status obviously.