You can lightly spray Nep leaves with a 1/4 strength solution of orchid food. This helps induce pitchers in the growing season. It works. Try to keep it OFF the soil. Don't bother putting it in the pitchers.

You can also feed them pelleted fish food. Don't over do it though. They don't need much. I give my smaller plants one pellet a week and rotate pitchers. Feeding them also keeps the pitchers alive longer IME.

....and some people apply Osmocote....