Hi funckychip,
I suppose if u can grow N. mirabilis, u shouldn't have trouble growing N. ampullaria. Like N. rafflesiana, the above mentioned nepenthes species along with N. gracilis do occur naturally in L/L condition in Peninsular Malaysia.
If a nep produce pitchers that's progressively smaller or stop pitchering, then something is not right. As growing condition is different for everyone, it's hard to pin point what's wrong based on the info that you've given so far....
Anyway, don't give up !!
