Problem

[eplants02]

What causes a pitcher to open immaturely?

 

[trashcan]

In my experience, stress of any kind can cause this. Temperature and humidity drops do the trick for me (in particular with a truncata as is probably your case).

See ya,
Pat

 

[nepenthes gracilis]

Generally it's a change in climate, can be light (burn the developing pitcher) or humidity in most cases which will stunt it and cause it to open up.