It's pretty erratic, indeed.
I'm booked for a bunch of CX F coming up, there was plenty of F out of JFK and then I went to book and it was gone, wound up going with ex-YYZ because JFK was only showing midweek departures.
Booked through to MNL but schedules changed and all of a sudden I was on 2-cabin equipment for HKG-MNL-HKG.
Checked daily for a couple of weeks and then this morning both flights opened up with 2 seats in F. ^
I've checked close to departure and seen the inventory cycling daily.
Almost no rhyme or reason it seems sometimes.