I feed ghost shrimp to my stingray, when the shrimp died my Tropicals would gladly eat them off of the bottom. Eventually, I threw a couple pellets in and one of the Tropicals just took them, the other Tropical examines them but doesn't eat them.
I think the main trick is if you feed them floating pellets, feed them off them something more tasty like floating fish or shrimp (anything they'll eat which will float) and then just keep throwing floating pellets in after a while. Once they get used to the idea of taking food in a certain condition (surface of the water or off the bottom) they should, but they may not, eat pellets from that condition. This is how I conditioned my Tropicals with the dead shrimp.
My one Tropical readily takes floating pellets also, the other one just examines them, but that's what the Tropical who eats the pellets now did also.
Repetition, allow them to be comfortable eating the food.