Some skills are impossible to rise to lvl 20 even when a person is doing the job all the time.
Problem is that after level up the skill is 0/x and it decays imminently causing level down.
The solutions are easy enough to implement in the patch or mod:
- make decay kick in with delay after the skill was last used or
- make skill down happen only after skill goes well below 0, lets say 5% of current xp required to level up, so if lvl 20 than skill down will accure at "-1050/21000"