I tried to use generals like bragg who had the training officer ability but it seemed like the units under his command never got any extra experience no matter how many turns went by.
That's the ability to turn 'conscript' units into 'regular' ones isn't it? If it is, it's worth checking into the components of units to see if you've got 'conscripts' there who'll convert up. Of course, that may not be the issue
edit: ah no, it's that one which exceptionally slowly raises experience whilst the commander and troops must remain stationary for it to have any impact that turn. Something like a year to get from no star to one star if I remember correctly last time I bothered to try to use it.