Under watering could be a possibility, especially with the way this specific plant seems to eat up all the love I can give it...never thought about that, my mind always seems to stick on over loving my plants (as I did that quite a bit on my first grow.

The drainage is pretty good imo, probably about 10 or so holes poked into the side of of the cup around the sides (bottom half) and 5 in a + pattern across the bottom. Soil mix has about 25% perlite in it to promote drainage (I usually see water forming a weak jet coming out of the holes after every watering). However, I usually check the root structure / growth every watering and theres usually some water droplets clinging to the inside of the cup around the bottom 60% of the cup. (edit: Before the watering)

I made the holes by heating up a nail and just pushing the heated metal point through the plastic (sheet and cup) prior to transplanting out of the baby dixie cups into those cups. (also did it away from the plants so they wouldn't be exposed to any of those nasty plastic fumes).