That's probably right. Protein will do it. Ancistrus species like a moderately protein-rich diet. Roughly 22% is good for them, according to Larry Vires on Planetcatfish.com.
Species from the Hypancistrus genus, such as the Queen Arabesques I have *prefer* protein, as they're primarily carnivorous.
Hikari makes a great "sinking wafer" for bottom feeders like Cories and Loaches that prefer a little more meat in their diet. I feed these to my Hypancistrus and Zonancistrus species now to supplement their diet. I'm thinking I'll put a little chicken in the tank for a couple of hours sometime soon to see how they react to real meat! I know Vampire Plecs like meat. I'm a veggie, so this would really be a big step for me!