strange, a single garrisoned general / admiral should level up after some turns.
the moment they level up go for generals for "capable bureaucrat " (red zeal) then "city governor" (blue) lvl 1
next level go for "city governor" lvl 2 and "political reformer" lvl 1
then the 3rd time the general levels up mx "city governor" to lvl 3 and "political reformer" to lvl 2
now you can keep him further to level up or (now comes the trick) disband him and recruit another general.
the old one ends up in Rome (the capital and adds all his traits to Italy PO and taxation , etc !!!!)
level up the new general recruit a new one send the old back to Rome, repeat, repeat...
by turn 80 you will have some 30 "state man" all giving the 12% tax plus to Italy , imagine 2000 base income Italy x 12% x 20 (!!!)
Same with Admirals, but they do not show the "city governors" while in fleet, they will when back in Rome ...
whatever they cost to recruit they will bring back ten fold in 10 turns...