It needs more light Fink. The exact same thing happened to me when I ordered some Green Dragons from this site. The leaf would form pretty fast and you'd see the tiny trap forming then it would start to get black and die. At the time they were in my terranium and I only had 1 20 watt fluorescent bulb. As soon as I added another double bulb 20 watt setup the next traps formed fine.
If you can place them in a sunny windowsil they love full sun.
The plant is kept in a sw facing window. I water each morning with distilled water. I have the pot set up on a slanted surface so that the water drains down. I did have all my plants setting in a constant supply of water, but mold was forming so I no longer do this. I would say the humidity is about moderate, not dry by any means. I do not see any parasites on the plant.
I just wanted to make a comment about mold. Mold is most often an indication of too much heat with not enough air circulation. Warm, stagnant air combined with a very wet environment can encourage mold growth. VFTs will do well in an open environment (in a pot on a window sill or outdoors). They don't have high humidity requirements like some CPs do, so a terrarium is not a "must". If you just prefer to grow your plant that way, just make sure it gets good air circulation, lots of light and not toooo much heat (like from an incandescent bulb) as an enclosed terrarium can heat up pretty quickly.