It's hard to remember unless I'm doing it. As I recall, after I had used my three FP+, I would choose today from the calendar and the park I wanted the FP+ in and what time I wanted it. The app would show me what was and wasn't available. If I was looking for something around 5 or 6, I would choose 5 PM and see what that turned up. If nothing, then I would choose 6 PM and check again . I would keep cycling back between those two times, just choosing alternate times continuously. Or you can just choose "afternoon" or "evening" and keep hitting that continuously.
My own experience is that it's easier to "modify" an existing Fastpass than to get a new one. So for example, after you've scanned through your 3rd Fastpass and are waiting in line for it, go ahead and select an easy to get 4th one (Spaceship Earth for instance at Epcot). Then try to modify it by continously refreshing as the previous poster said. You might then pop up a couple of different times for the ride you really want, although by trying to get it originally, you can't get anything.
Thank you both!!

