My passifloras flower a lot, but seeds never appear to mature. What are other ways to propagate them?
Do you know what kinds you have? If not do you have photos?
Some are easy to propagate by cuttings--in fact many of the commonly available kinds are.
Often passionflowers are self-infertile, so you won't get fruit/seed without a plant of a different clone around. Sometimes a different species will work. And sometimes you don't have the right insects (or birds or bats, I suppose) around and so you would have to hand pollinate. And some hybrids are sterile--you will never get fruit.
There are techqniques besides cuttings and seeds, but those are the most straightforward and for most plants they suffice.