I assume you're playing Black Pits to have reached level 10 as a fighter?
Well, you're at the level cap. You can only go as far as level 10 in the Black pits, so dualling was wasteful and you should reload to before you did so. I believe that you would be able to eventually reach level 10 in Thief if you had dualled at level 9 fighter, but someone else should confirm that.
Either way, this isn't a bug, it's an intended feature. You've run up against the XP cap. In the future, consult xp tables to determine whether or not your intended dual-class will have enough time to regain inactive levels under the xp limitations imposed upon you.
Comments
Well, you're at the level cap. You can only go as far as level 10 in the Black pits, so dualling was wasteful and you should reload to before you did so. I believe that you would be able to eventually reach level 10 in Thief if you had dualled at level 9 fighter, but someone else should confirm that.
Either way, this isn't a bug, it's an intended feature. You've run up against the XP cap. In the future, consult xp tables to determine whether or not your intended dual-class will have enough time to regain inactive levels under the xp limitations imposed upon you.