Question

Nepenthes Species   my Nepenthes won't grow pitchers, they are not dying, the leaves are not withering but they've ceased growth altogether. I've been told that terrariums are not required but that the plants do respond better to higher humidity. this is why I have place a large plastic bag over them with holes punched in it. they are not sitting in water either, I have them sitting on a plastic container to keep them out of the water. All four of them are in a south facing greenhouse window that receives very bright direct sunlight,which is why I have placed them on the bottom tier. two are intermediates, one pure and one natural hybrid and the other two are highland hybrids and if it helps their genotypes are chaniana x veichii, talangensis x ventricosa, (inermis x singalana) x mira, these first 3 came from your greenhouses and the last one is a pure N. alata as far as I know. Sorry for the dreadfully long question but I thought it would be better to give you as much info as possible. I can't for the life of me understand what I am doing wrong, I have also been keeping a watchful eye on them for mold growth and so far there has been none whatsoever and I have been watering them every few days. Thank you for your time.
         -Brad

Answer
First, remove the bag from your plants.  Under no circumstance should you put a plastic bag or any sort of enclosure over plants that will be exposed to direct sunlight.  This is a very good way to kill them.  The lack of air circulation will keep the heat in, and your plants can die from overheating.  Even with holes in the bag, you shouldn't do this.  We do not at all recommend this type of growing conditions for any plants.  It's too dangerous, even for an experienced grower.  The photo on the left is the perfect method of growing them, and the method we recommend.

Second, you may need a heavy dose of patience. You didn't mention how long you had them, but sometimes Neepenthes need a month or two to acclimate, depending on your growing conditions.  
In some extreme situations, you may need 6 months.  

Third, we are also coming out of winter.  It's very normal for Nepenthes to stop producing pitchers in winter because of the drop in sunlight.  No matter how much humidity or warmth you give the plants, if the lighting isn't optimal, they won't produce any pitchers.  This is completely normal, even for us.  So your plants may need more time to recover from the winter season.

I also don't know if your box window faces north, south, east or west.  The leaves look shiny and healthy, so it just might be a matter of time.  Judging from how your plants look, I don't think humidity is really a factor.  

I recommend watching our monthly video podcasts.  We talk about Nepenthes that don't produce any pitchers.  It's a common them in winter time.  (The March podcast talks about this issue.  Even for growers in Hawaii where humidity is optimal, the shorter days sometimes makes their plants stop producing pitchers.)  I know we're in early spring, and even for us, we have many Nepenthes not producing any pitchers.

So take the bag off of your plants, make sure the soil is moist, and make sure your plants get direct sunlight for a couple hours each day, with bright indirect light during the rest of the day.  And then give your plants time.  

You might also benefit from watching Grow Carnivorous Plants, Volume 3.  We show you what type of window to grow your plants and proper ways to increase humidity that won't potentially kill your plants.

http://www.cobraplant.com/videos
http://www.cobraplant.com/DVD

Good growing!
Jacob Farin