My experience has been that it is typically easier to find them earlier in the day. My game plan, should I need to get a hard to find FP for my 4th of the day is to go to my first pre-booked FP as soon as the window opens. As soon as I tap in, I then start trying to find a better time for my second pre-booked FP. Often, I can move that one up enough to go straight from the first ride to the second one. I then do the exact same thing to move the 3rd pre-booked FP up, with hopes of getting off the second ride, and going straight to the 3rd, and then as soon as I tap into the 3rd, I start looking for the ride I want. If I find it at ANY time of the day, I take it, then go back, and try to modify it for a better time. Any time I see it at a better time, even if it is only a 5 or 10 minute difference, I take the better time, and then try to modify it again. In this way, I am usually able to walk that fast pass back to a time that is reasonable. I may start with a FP for FoP at 8:30, and when I am done it is for 3:45, and we only have to wait half an hour before the window opens up.