The potential of a trainee doesn't affect training speed, but there could be several factors as to why he had 11 pops (which isn't to bad, that's a little under one per week).
Training speed can be affected by the age of your trainee. 18 and 19 year olds train fastest, but they won't train as fast as they get older. Another factor could be your trainer. If you have the lowest level of trainers then chances are that you are missing out on some valuable training. Also if you one position train your trainee then he will pop much quicker as opposed to 2 or 3 man training. A factor that some people don't think about is height. If he's a PF/C, but is only 6'3" then he won't pop as fast in INSIDE skills as a 7'0" trainee. Same with a PG/SG, a player won't pop as quickly in OUTSIDE skills if he's 6'8" as opposed to 6'0".