Probably not a problem with the Simulator. The 6'4" 19yo on your team has role player (2) potential. And with a SF salary of $4.2k he is likely capped. This would account for the slow-down in training you are experiencing (in comparison to the model).
But the Simulator is designed to let you know when this is the case. Take a look at the CAP number to the right of the training results. It turns red when a player is near his potential cap. This indicates that the model the Simulator is based on no longer applies (because it does not model the cap slow-down).
Even if his CAP number isn't red, your player may still be capped. Like you said, you don't have a good idea of the sub-levels on his skills. And so if enough of the other skills have decent sublevels, he will be closer to the cap than the simulator shows.
Hope that helps.