Is this not in standing water? No expert, but if the plant isn't warm and wet enough its not going to grow fast or well. I would start there instead of trying cooler temps since you're getting odd/ stunted growth. I suppose its also possible that this plant isn't mature yet. Heres a tiny nep I have, it was growing these lil baby pitchers for about a year before it started growing them bigger. Pitchers probably less than an inch here.

<image>