Do you get any chill hours? I think cherimoyas need some minor chill time maybe 100+ hrs
Jeff,
According to California Rare Fruit Grower's website, cherimoya requires about 50-100 hours of chilling hours; then I looked up your average yearly temp. for Kona, it shows a pretty much steady high 60F's to low 70F's for your night time lows... So, even at 1800ft elevation I don't think you could get to low 45F to reach the chill zone.
So, as yourself and Brad suspected, this could possibly be the reason of you having difficulty flowering cherimoya at your location. Good to know your atemoyas are doing well there, though...