After playing around a bit I remembered one reason I never played the original past the first levelup or so.
How can it be that a fighter with max vitality only gets ONE hitpoint on levelup!?! :(
Maybe the devs could add an option to make leveling less frustrating. Otherwise I'm inclined to ask for a refund.

Maybe it might help if I'd explain how level up HP gains work in early Wizardries (1-3 and 5, plus some Japanese spin-offs).

When you level up, the game re-rolls your hit points from scratch.
* If the roll is higher than your max HP, it becomes your new max HP.
* Otherwise, your HP only increases by 1.

So, what most likely happened is this:
* You got a very good HP roll on the previous level up.
* On your current level up, you did not beat that good HP roll, even with the extra HP from the previous level up. As a result, you only got 1 HP.

Because of the way this works, getting only 1 HP won't hurt your character in the long run; next time you get a good roll, you'll get *all* the HP you missed.

Note that 1 HP gains are also to be expected in a few other situations:
* Your character lost Vitality at level up. Once you regain that point of Vitality, expect to get a decent HP roll.
* Your character lost Vitality due to being the target of DI or KADORTO.
* Your character class changed recently. Expect 1 HP gains until your level and Vitality catch up.
* Your character lost a level without losing HP; this happens if you cast HAMAN or MAHAMAN. (If the level loss is from an enemy's attack, you lose max HP loss is proportional to the level loss.)