yes, it can be hard to get them to eat. Lots of em won't come out and attack a pellet/algae tab/whatever during the day with the lights on. So those will eat at night with the lights out while you're asleep--they're highly nocturnal usually. Most of 'em are wild caught, so they may not be used to eating what you're feeding them. Then they stress from shipping, so they may sit in your tank and not eat for the first couple weeks while they get acclimated. They don't usually eat like cichlids do, where you drop the pellets in and they just suck em down in a minute or so. Pleco's really like to graze all day long.
And then there's hundreds of different types of Plec's with very different diet and behavior. It's complicated.