Solved

Training at different speeds.

I've got two drivers (ones a backup) but they both train at very differnt speeds. The better one loses about half their health before they improve while the worse one improves after losing about 20 health.
Anyone know why that is?

Seeing ur team, Palmer will train slower than Evans since he's much older. 29 versus 17. I cannot see ur reserve. But this is the main reason. 

Oh ok thanks.
Thay will be it as i didnt know their age had any relevance to training.

Matt, drivers' training speed decreases when their level increases. The closer a driver's level comes to your own level, the slower his progress will be with every training session.