Fruit set variations
I would comment re attempts at setting hybrid fruit that I have noted the following variations:-
1. No fruit set.
Some passionflowers can self pollinate but this is variable even within one species, e.g. some P. incarnata clones are thought to self pollinate & most can't. Dr. John MacDougal comments,
'I'd say more than 90% of all wild species of passiflora are self-incompatible and require cross- pollination. Some species vary within their own species on this, with some populations as selfers, derived from the rest of the species that is self-incompatible. This is rare. See it in P. suberosa clones, P. coriacea, P. cuprea, etc. Some species have some minor variations and "systems" that are actually so very complex in the subtleties of gene-flow, that only God can know the immune systems of these plants! You know, some plants have weak styles, or act more male at some points in their life than others, even the age of the plant can affect this. I have seen some weird things happen with those flowers! But overall, they need cross-pollination should be your first guess.''
Learn more about self pollination.
Some species always need an unrelated clone e.g. P. hahnii and indeed P. caerulea. Many people think it can self pollinate because it fruits so readily but the pollen will always be brought by bees from a plant nearby. Some, such as some of the P. murucuja clones, are not self fertile, but when foreign pollen is applied they somehow set fruit without the pollen giving any genetic input. So you can think you have hybrid seed, when in fact you just have selfed seed. This phenomenon has been observed & much discussed with regard to a number of Passiflora & other plants. It is called 'apomixis', the ability of some plant species to reproduce asexually by their seeds. Usually all the offspring will be identical clones of the mother plant, but some genetic variablity may occur as there are different ways that the normal reproductive process can be short-circuited.
Many hybrids used as the female parent are sterile & will either not set fruit or like P. 'Sapphire' produce empty fruit. P. x violacea hybrids however will produce about one viable seed per dozen fruit, so don't give up too readily. Further, for reasons of genetic incomptability, many hybrids are poor pollen donors producing little or no pollen, often with deformed anthers. That does not mean that it's not worth trying though. Some crosses are impossible if the plants are too different genetically. Some crosses are notorious, such as P. vitifolia x P. caerulea. This will readily set fruit, the seeds of which will produce vigorous healthy hybrids which very rarely, flower. P. 'Michael' is the only cross of this that I have known to flower. Even repeating well known successful crosses can produce very disappointing offspring.