... they already found other more interesting things in the tank to feed on? ...
The same can be said for Amano Shrimp, Cherry shrimp , and even Ramshorn snails. Algae crew members are always going to fill up on prepared food BEFORE eating algae. It is basic survival. The prepared food is a temporary bonus, the Algae is a constant.
Starve off the tank until it appears completely clean of algae, then wait another day before you offer food. There will still be algae you can't see without a magnifying glass.