This and the other symptoms you describe definitely sound like lack of
light. If I may be so bold I think what Gordon was saying about VFTs
needing several hours of direct sun every day could be taken as "VFTs need
the equivalent of several hours of direct sun a day". They can get it from
light bulbs or from the sun. Sunlight has the advantage of being
full-spectrum. And besides, you can't beat the price ;)
Good growing,
Phil