I have constitution of 18, when i raise a level i always get 1 or 2 hit points, even on easiest when you should get the maximum. How do i fix it? And what is the problem?

I guess you are multi class or dual class. Of course you don't get for each levelup your full hitpoints when you level up twice as often as the usual character. Others are you could be a mage or alike, who usually get less hitpoints/level, even with the same consti as a fighter.

I am a kensai, not a multiclass or a mage.

The only thing I coul think about beside this is that you might mix up hitpoints with THACO or savings or lore or whatever. You really should get like 4 hp/level minimum with 18 consti und below level10.