I recently managed to improve my constitution by raising it from 18 to 19, and so I expected to get a hit point bonus. But I didn't, and I'm wondering whether this is a mistake on the game's part. Does anyone know whether I should get extra hit points from this?

Depends on your class.

A fighter gets +1 hp per level for each point of con over 14. So going from an 18 to a 19 should get you your level in hp, up to a max of 9 as after level 9, you only get 3 hp per level, no more con bonus. The same applies to paladins and rangers.

However, all the other classes are capped at +2 hp per level, from con. For them, a con over 16 doesn't help hp at all.

Mind you, that's all second edition rules... but that's what BG is.

Thanks Axterix.

I guess the problem would lie in the fact that I'm a mage...so I guess that means I don't get a hit point bonus.