Actually, I think the no. 1 symptom of overfertilization is not pitchering. Have a look at the Nep guide on the CP FAQ. You may want to try switching to a low-mineral-content water for a few months. Neps only produce pitchers to catch bugs when they are nutrient-deficient; if you're fertilizing it, then it doesn't need to be carnivorous and can grow better by focusing on growing big leaves and stalks rather than pitchering to find food. Neps get all the energy they need from light - the occasional bug is sufficient for their vitamin/mineral needs.