I have a couple of questions regarding your diagnosis.
1) If the problem is from mites, should I be able to see the little
   critters?  I have not observed any insects in or around the plant.
2) I have been feeding the plant ants and small sow bugs since I got
   it.  This has only stopped recently.  The pitchers have been dying
   off faster than the plant can replace them.  Considering this, is
   it still likely that the plant is nitrogen deficient?  Does the
   use of dilute chemical fertilizers cause any stress to an already
   weakended plant if it turns out that it is not nitrogen deficient.
3) I probably should add some more info on the plant's environment.
   About a month and a half ago, I was having the same problem with
   three small (1.5 inch diameter) N. ventricosas.  (Keep in mind that
   in the same aquarium right next to these plants I'm mentioning I
   had a fourth N. Ventricosa that was/is RAPIDLY increasing in size.)
   I took the two plants in the poorest condition and placed them
   outside.  At first they hated it even though I misted them down
   about ten times during the day to help them adapt.  I am no longer
   misting them down.  They are growing now, producing good dark
   green leaves, though they are yet to pitcher.  The third unhappy
   ventricosa still in the aquarium has picked up, but the leaves are
   still a little pale, though not as bad as before.  I would be eager
   to move the N. madagascariensis outside, except I am not going to
   be around during the day to mist it repeatedly.  In addition the
   temperatures outside are much hotter now.  In any case, there is
   still the problem of what to do with the plants outside during
   winter.
   I just don't get this.  I have two rapidly growing N. khansianas.  A
   very happy N. ventricosa, a happy N. Rafflesiana, and an N. Alata 
   seedling that is doing fine together with various other CP in the
   terrarium.  The problemed plants were in the choice spots in the
   terrarium.  All my Nepenthes have good coloration in their pitchers
   (except my Khansianas which have green pitchers) including the problemed
   ones.  Do some Nepenthes show a sensitivity to spectral imbalance
   from flourescent lighting?  I hope I can get to the bottom of this.
Thanks for the help,
--Zachary--