A number of Tacsonia like P. tarminiana above and P. antioquiensis can self pollinate. Tacsonia, which are principally hummingbird pollinated, are also known for hybridizing in the wild. Their pollinators are of course much fewer in number than bees. They may accordingly have had to accept that the floral tube length which attracts a hummingbird to them will also attract them to other Passiflora species of a similar tube length, & indeed to other plant species altogether, such as Brugmansia, Datura & Fuschias. So in some cases they cannot even afford to depend on pollen from another clone of their own species & have to allow related species to cross pollinate with them rather than not be pollinated at all. So perhaps there are shared glues & binding initiators in different Tacsonia species. It is worth noting that they are very invasive in some countries with no hummingbirds at all and I don't think anyone knows what is pollinating them there - if anything. It could be the wind or as the flowers close up they may fertilize themselves.
Other Passiflora are visited by pollinators such as bees, which may be generalist at colony level but are often specialist at individual level, visiting particular species of flowers, hence orange blossom honey etc....and presumably somewhere Passiflora honey? Nevertheless even specialist bees may be forced to diversify depending on flowering seasons & hive proximity. So perhaps these species each have their own special glue formula for specific pollen adhesion, explaining how the plant selects the pollen it wants from whatever is deposited on the flower's stigma. The physical shape of the grain against the stigma producing a tight fit may be relevant.
There is also the widely observed phenomenon whereby 'alien' Passiflora pollen, that is not genetically compatible, will in some cases trigger selfing, somehow making the stigma cells receptive to binding with the species clone's own pollen. The complexity of what is going on in these & indeed all flowers is remakable. Also where is the defence against alien pollen....on the stigma or do some alien pollen tubes force their way down the style only to be defeated later? I recently tried to cross P. trisecta with P. caerulea & P. 'Purple haze'. In both cases I had fruit full of seed & juice..but the seed were empty.
Rain often interferes with pollination. The pollen may be simply washed off the anthers or stigma. Also even if it isn't washed off water causes pollen grains to explode by sheer osmotic pressure, as observed in Passiflora incarnata by the Englishman, Mr. R. Badcock, Esq. in 1748. Nevertheless stigma cells initiate binding by water release to the pollen grain so perhaps the osmotic pressure is reduced by binding or is used to explode the pollen tube downwards into the stigma.