Naranjilla (Solanum quitoense)

Does anyone here grow this plant? I recently got a couple of small thornless ones from Exotica Nursery. I have had a large thorny one for a couple of years, and every spring it makes the flowers, but I never get any fruit. They just wither & fall off. Then it makes more up the stem as it grows, but no fruit. It seems healthy enough, just won't make fruit.
Any suggestions on how to get fruit? Do you know if they prefer sun or shade? Seems like mine tends to get burned leaves with sun so I keep it in partial shade. Thanks.
Marcy

I have no personal experience with these plants, but found this to be interesting reading:

http://www.hort.purdue.edu/newcrop/morton/naranjilla_ars.html