Sometimes you have to wait 2 hours for another pass, sometimes 20 minutes. Read your fastpass ticket.
Also, when the parks are very busy, all FPs for a particular ride may be all gone (issued) as early as 10:30 am. This used to happen quite often on TestTrack.
Two week ago I returned 3 hours past my "window" and a CM said (sarcastically) "We're quite a bit late, aren't we?".
Later in the day, I mentioned this comment to another CM who was assisting guests at a different fastpass machine. She apologized, was amazed that the other CM even made that comment, and issued me a couple of extra fastpasses.
To confuse the issue even further - sometimes when the fastpass machine is malfunctioning or extremely busy and a CM issues you a fastpass manually, that manually issued ticket will not say "you can get another fastpass after xx time". Which means - you can go to another ride and get a fastpass right away.
This happened on Exp Everest 2 weeks ago. I received my EE fastpass, manually, from a CM at the machine - (seemed like 2 million guests waiting in line) and went immediately to the safari and got another fastpass.