You might try feeding unblanched veggies (also, have you tried a variety? Leafy greens, zucchini, broccoli are all potential choices). Blanching, in addition to making veggies softer, also makes them sink. I feed blanched zucchini to my applesnails, as they cannot eat it otherwise (applesnails feed much easier from decomposing food). However, any pleco who's diet includes wood is perfectly capable of (and may even prefer) raw zucchini. I have a royal pleco (a panaque known for wood-eating habits) and he won't touch blanched, but loves it raw.
The trouble with raw zucchini is that it floats. I use a metal fork (watch out for rusting) to hold it down where the pleco can eat. There are other ways to weigh down veggies, but overall I find a fork the most convenient.