no, he was prominent when I purchased him and he was supposed to pop to prolific
10 to 11
Training is one of the things in Buzzerbeater that doesn't malfunction without sending a massive alert to the devs. So the one of the things that's almost certain is that there was no training malfunction.
My best guess is that you confused proficient for prominent (two words that look similar and appear in almost the same color), and then it seemed like the game indicated a pop where there was none.
It is also possible that the player trained with his previous team, if you purchased him after their (and before your) training update. This is also an easy mix-up, since green arrows are visible on player pages, but not in the transfer list. In this case, the player is not going to train twice.
(of course, there is also the consideration that not everything pops in 1 week of training, but this is not the issue there, since I assume from your post that youyou observed a green arrow next to that player's skill).
"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."