I recommend repotting those two plants with nepenthes media.
The two plants are very tolerant of different potting media's expecially xVentrata so don't think all nepenthes will grow in normal black soil.
Some will grow but slowly and small pitchers.
My xHookeriana grows very slowly in black soil,(hardly a pitcher a month) and after i've repot it with sphagnum moss,it seems to like it's new media very much and start growing very fast.
I hope you learnt a lesson here Ken!
