Your pleco problably wasnt used to eating them, and realized it was food. Most pet stores usually dont feed them anything but the algea in the tank. Mine did the same thing when i first got it and now it goes crazy on them.
i have 2 plecs ( common plecs ) the bigger 1 dont seem to realy eat any treats apart from cucumber but the smaller one eats just about anything but with the bigger one i only realy ever catch him eating when all lights are out even with the moon lighting on he dont realy eat just comes out for a swin try and catch your one n the dark im sure he does eat just not when your looking