My experience feed training RES is that once feed hits the bottom most of it will go uneaten. A very slow sinking feed would be better, movement seems to be a feeding trigger and once it hits the bottom and is no longer moving it normally doesn't get eaten. Last time I feed trained RES I had 4 or 5 RES that would pick pellets off the bottom of the tank, the other 80 of so RES just ignored the motionless feed on the bottom.
I had a smaller number of RES in an aquarium but basically observed the same thing as Shorty. Once the feed hit the bottom they lost interest.
I would go out and get snails out of the pond and put in for them to eat. When a snail moved the RES would race over to it, hover above it at a 45 degree angle staring at the snail, but would not attack/eat it unless it was moving. If the snail sat still long enough the fish would lose interest and go away. I never knew if it was just not hungry or if it needed the snail to move before it ate it. Many of the snails were too big for the small RES to eat also so that could have been an issue. Sometimes they would bite at the exposed meat part of the snail before moving on.
I did this when I bought some RES to stock my forage pond. I just brought a few inside to watch them for a few weeks.
