Passion Fruit Makes Empty Fruit

I have a question regarding Passion Fruit. About four years ago, I received seeds from Richters for passion fruit (S4441). These have been growing in the greenhouse and doing great. However, the fruit they are producing are empty. The fruit appear but are only small and basically just the shell and a bright orange in colour. Is it something that I am doing? Or did I not get the right ones. I know that some varieties do not produce fruit, but I thought these did.

You are right this is the species that is supposed to produce nice fruit: Passiflora edulis. I have never heard of this problem with respect to this species, but it is a fairly common occurrence in other plant families, such as nuts. In nuts this happens in young trees, in trees that do not have another clone as a cross-pollinating partner or in years when conditions for pollination are poor such as very rainy conditions during flowering and some plants have a genetic fault that prevents normal fruit development. My guess would be that passiflora may have similar problems.
In any case make sure the plant does not dry out or is starving, before trying the option of growing more plants. Since many plants will not produce fruit unless it contains viable seed, this may be the lack. If you at present have only one plant, do try to manually pollinate it, because you are not likely to have any natural pollinators in the greenhouse.
If the plant does not start producing proper fruit as it ages and you tried self-pollination and faultless culture, try getting two plants -grown from new seeds and manually cross-pollinate between flowers of two different plants.
If you get fruit with the two new plants then you can just throw away the first plant, because it has a genetic problem.