Incarnata is typically self-incompatible. I assume you pollinated it with itself? The one fruit is likely due to an insect visiting from a nearby incarnata, which are likely to be growing wild all around your area.

My Incarnata is Growing inside on my Windowsill... So I Don't have a lot of Choice in what I use for mine... I'm Growing for Fruit, Luckily Mum has a caerulea Constance Elliot... So I Used that and now have Two Fruit out of Three Flowers... So I'm Happy...

aww! darn!
I doubt it was from another incarnata around here, since I would have seen it... it would have to be pretty far away. Maybe it was just an accident lol.
And that's the only passiflora I have (besides, I don't want hybrids)

I Should of said that I'm Growing Fruit for Eating not For hybrids
Sorry...

I thought P. caerulea wasn't edible?
I'd like fruit for both seeds and for eating.
oh, and I DO have other passifloras (P. edulis and P. lutea) but they're not flowering.

P. caerulea is edible... But the Fruit is better in Pies with Apple than to eat on their own... Hope that Makes sense...