What I have observed with any feed is if the pellet the fish takes is hard and it is a little on the big size for them they will spit it out (often with a larger fish right behind them that takes it in and keeps it). If it has been in the water long enough to be softened they will retain it.
Very similar reactions from my relatively young fish. Biggest pellets are sometimes spit out by smaller fish, though they eagerly take smaller pellets so it obviously isn't a matter of taste. Eventually the bigger pellets soften and are eaten, sometimes 20 or 30 minutes later.
