I just went through this twice - once for a 4-day trip at the beginning of September and just now for a 6-day trip mid-to-late October ... It is getting harder and harder!! Oh, also did it in June for a 6-day and had no problem whatsoever getting FP+ for FoP on our first day and later in the trip at the times we wanted
I was only looking to book fastpass for FoP - we did Navi in June, and I personally could care less if i go on it again - I'll always settle for trying standby - We are a party of 2 so there may be more availability for our small party size vs. family trips
September trip ... closing time of AK is 9:00 pm
1st and 2nd day nothing - Saturday and Sunday
3rd day Monday EMH AK am - had to settle for night-time FP for FoP
4th day our departure day wanted mid-day FP for FoP - and that's what I was able to get - all morning fastpass were gone
October trip ... closing time of AK is 7:00 pm
1st day Thurs
MNSSHP day - none
2nd day Fri MNSSHP day - none
3rd day Sat - none
4th day Sun MNSSHP day - late-afternoon onward, fair number of slots
5th day Mon EMH AK am - grabbed first slot available 12:05 pm, wanted early morning and so did everyone else (plan was ropedrop FoP and 2nd ride right after w/fp) - pretty open availablity otherwise, kept trying to move the fp up and by later in the day only 4:30 pm and later was available
6th day Tues departure day - could have had any slot I wanted - chose 9:30 am, checked this morning and still fair number of slots with earliest being 9:30 and lots of mid-morning slots are missing
Lots of details - hopefully they help someone - I think we are seeing the difficulty of booking these FoP becoming even tighter than during the summer because the closing time of AK keeps getting earlier and crowds increasing for the fall months. In June AK had a scheduled closing time of 11:00 pm vs. October's time of 7:00 pm - that is an awful lot of lost FP+ slots!!
I think it may be easier to get an FoP fp for MK party days.
I think it is harder to get an FoP fp on AK EMH am days.
I would not count on getting a fastpass on 3rd day or earlier.