Depending on what type of Passiflora it is this could be normal as some shrivel before dropping. If you mean edulis flavicarpa however, first open the fruit and see if it is full of juice & the seed look ripe i.e. black. If the fruit is immature inside it could be too much Nitrogen in the fertilizer..or more probably fruit flies which lay eggs in the young fruit causing them to fail to ripen properly. Look carefully for entry holes or scars..plus larvae!

Thanks for your response Passionflow. I don't know what the name of this passionfruit is - Just an ordinary yellow fruiting type which does well in the tropics. Inside the fruits are full of white immature seeds. I haven't noticed any fruit fly entry holes but will look more closely. I've been feeding the plant loads of Miracle Grow. Maybe I've been overdoing it!?
Thanks again
Gemmas

Miracle grow is too high in Nitrogen so that could be your problem. Just stop feeding and hope.
Myles

Hmmm... So drop the N during fruit production? I did not know that.

Maybe that's why a bunch of my flowers shriveled and fell.. Or too much heat, not enough water.. Or I looked at it too much.. Sometimes these plants seem a bit fickle, but others stay happy no matter what I do.

Can be quite confusing.

Thanks for your advice. I stopped the nitrogen and all new fruit are growing happy and healthy!