Welcome [img]graemlins/wavey.gif[/img]
This generally happens when you are killing low level monsters in a game difficulty beneath where you should really be playing. You will gain some exp. but it will be a looooooooong slow process to level up
- especially when you have to split the exp between your characters.
If you are still playing Novice, I would suggest you start a new game with your wizard only and play that until he reaches level 20 then you can start a new game with both chars at Expert difficulty.
A small hint - after a while, most players go solo, and develop one character at a time
Hope this helps.
Btw, are you any relation to Lissa whose stats I used to chase on the Diablo Forum