Stop feeding them anything else. What's your entire stocklist? Odds are you can take a week to focus solely on this one food. Just put in a fresh piece every night. It can stay 24 hours easily. Then, the next night, replace it with a new piece. Repeat. Feed them nothing else. It's all in the technique. I've gotten all my plecos to eat zucchini this way. They just won't starve themselves, and once they have been forced to taste it they'll never refuse it again. Then you can go back to the usual smorgasbord of massivore and algae wafers and they'll take the zucc at will. They will also associate the anchor you use with edibles so you can start popping asparagus and peppers on there, too.