I agree with Adam. In my experience with plecos they don't always go after the pellets or waffers right away. It's like they don't know they are food. Once they try them, then from then on they go crazy over the food.
I also make it a habit to drop my pleco food in the same spot, and my pleco heads there to check for food drops.
Generally, when they get hungry, they get more aggressive in their search for food.