All these pops (or lack thereof) might be caused by a lack of balance in development of the players.
I've had a guy with proficient IS and strong ID pop in consecutive rebounding trainings from mediocre to average and then to respectable. He's neither extraordinarily tall nor extremely young.
Likewise, if rebounding is several levels ahead of other skills for the 22 year old guy, that might slow its training down.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."