Yes he trained 100% during 3 weeks and this last week he made 44(but thats really close to 100% of training...)
I've read somewhere in the forums that the penalty of not getting 48 minutes is substantial. I don't have any data to confirm that statement. Maybe someone will confirm its validity.
Don't worry about the height cause in my experience with JS training height doesn't matter at all. As example, my 19 year old player
(16969339) received 6 weeks JS training SG/SF (3 last season and 3 this season) and improved 7 -> 10 at JS. If he improves next Friday he'll have pop every 2nd week.
The age effect might be a problem though. This 22 year old player
(12173771) is 3 weeks without a pop in JS training for SG/SF (full 48 minutes). Last season (as 21 year old) he needed 3 weeks in JS training for SG/SF to get from one JS pop to another.
My trainer is level 6.
P.S. Also, make sure that the level of JS is not too far away from the related skills cause it also affects training speed.