After you capture a lord and throw them in your dungeon it is highly likely they will have magically disappeared within a week. It renders taking lords prisoner pointless as there is no strategic gain to imprisoning them if they can simply leave and return with a 300 man army a few days later. Defeating a lord and taking them prisoner should weaken an enemy faction but currently it does nothing. The fact they can escape from your party is understandable but the fact they can just as easily leave your dungeon is absurd.