Hmm... assassins

Sometimes it is hard to understand - for example i have one with 4-5 "eyes"
and he manages to kill Mongol 8 star general and receive no trait increase. On the next turn he fails to kill another and receives "trait decrease" to 2-3 levels! I mean, WTF ?! Sometimes it took ages to build up his skill and now i should start everything from a scratch. Save/load isn't a remedy in this case - you have to wait for another turn.

Also I have noticed a funny thing in my last HRE campaign. After city of Novgorod was seized by my troops, I was able to build 4 level assassins from the start (due to presence of Coaching House). HOWEVER, when a Gunsmith was build in this city, my assassins started to appear only with 2 levels and sometimes with "Hand-gun prototype" ancillary (not sure if correct), which gives exactly +2 to assassination skill.
That means some buildings can decrease assassins (also merchant/spy/diplomat/priest) skill?