Harry
The doctor is rated 5 stars. Thanks for your reply!
Check their stars, from memory, when they have 5 gold stars, they should increase by about 9 every time they train (1x), and if you train them past that, they are higher level than you and the stars turn red, where they then increase by 3 every time.
Doctors do not have an effect on how well they train, but on how much health they spend doinf so.