Hi,
Thanks for writing. Passionflowers do best in a spot where they get full sun and well-drained soil. Most passionflowers would rather be too dry than too wet, so be sure you're not overwatering it by letting the top inch or so of the soil dry out before you irrigate again.
Do you fertilize? If you have lean soil, a shot of general-purpose fertilizer can definitely help your plant put on more growth. If you're already fertilizing, you might try cutting back --- too much fertilizer is more harmful than not enough.
Passionflowers do bear fruits (passion fruits). Some varieties are edible and delicious; others are not edible. If you remove the fruits, your plant should do a little better, as well --- it'll put more energy into growth instead of producing seeds.
---Justin, Senior Garden Editor, BHG.com
Another reason it may looks sparse, if by sparse you mean the leaves have disappeared, is that it is hosting fritillary caterpillars. Passiflora is the host plant for a number of different species of butterflies.