Why won't some players I make available for U21 team play?

[mitchell.sam]

Playing in England for the first time, so a little unsure about how reserves work. I have some players that are not regular first team players that I want to get time in the U21 squad. I make them available ongoing, but they never player. Other players that I make available do play. There doesn't really seem to be a difference in the players that would indicate why this happens. I even tried getting rid of all the other crappy U21 players, and now greyed-out fake players are played instead.

Why does this happen?