I went and looked through my
Nepenthes bible: Pitcher plants of the old world vol. 1&2
By Stewert Mcpherson
It seems that
N. argentii is yet another one of those carnivorous plants that grows
in a high alkaline environment with serpentine rock.
I will make a few quotes:
"Sibuyon Island... on the summits of Mt. Guiting—altitudes of 1400-1900 m. The upper slopes of these peaks are completely exposed,
wild and windswept, and consist of dense swathes of severely stunted upper montane scrub, to 40 cm tall,
punctuated by razor sharp serpentine protrusions, boulders and rock falls.
Nepenthes argentii
has evolved to survive this extreme habitat as one of the smallest of all
Nepenthes,
the rosettes of mature plants rarely exceeding 25 cm in diameter."
He also goes on to say:
"....grows in shallow, nutrient poor, rocky substrate, nestled amidst surrounding vegetation and rocks,
sheltered slightly from the worst extremes of the harsh climate—
"plants tolerate strong usually strong direct sunlight..."
"— Highland windy habitat..."
I would think that maybe your plant is not getting its basic requirments, (no pun intended) rather than acidity.
Maybe try crushing up some serpentine rock— thats what i do with my more alkaline plants.
Hope this helps