I assume getting 100% training (1.0 skill up a week) requires a perfect hight, age, trainer level, one position training and maybe even higher skills other than the training skills.
So, for example if you train rebounding, you may need an 18yo guy who is like 2,24m/7'4" (don't know what the actual maximum/minimum hight is), a level 7 trainer and and maybe at least one related skill above the current rebound skill (ID/IS/SB).
Anything else will slightly decrease training speed. That is my guess, but it could be interesting to have a discussion about it.
Besides, it could be that your trainee has a high sub-level at the current training skill. For example if your trainee's Rebound-skill is at 7, it could actually be 7,9, hence, you can have a visible skill up for several weeks without having optimal conditions.