Were you were expecting human behavior from fish when eating (or anything else)?
All my plecos aren't what I consider good hunters, but rather opportunists, finding what they can, even after they continue to get the same type of meal for years. In the wild the water is moving swiftly around them, so they are probably going more for what they can get now, than what they can sit back and enjoy. Their swishing around is them looking for more food, and one of the reasons they are best to be kept alone as they are easily beaten to the food, or at least one should make sure they are getting food if kept with other types of fish. As the tablet breaks up, I'm sure they are finding small bits of it, but there is a bit of waste too, so there is a need to keep the tank clean.
Anyhow, it sounds like your are doing their normal thing. They move to the food, eat some, maybe get nervous and swim away, come back s-l-o-w-l-y searching for more, get scared away, come back, eat some - repeat. They pretty much have all day to do this, so what's the rush for them? This is what it seems to me, imo.
They then spend ages swishing the dispersed food out of their caves - so it's not to make it go where they can eat under cover.
