Due to the lack of information or save file this will be a list of questions.
Which version (can be seen in bottom of main menu) of the game are you playing? This helps with assessing code.
Have you ever applied any mods or altered the game's code or data? (Edit: If so then please list them exactly.) This exact problem seen for those who had the wrong version of Leo's mod installed.
Did you return to the mansion before attempting to submit a slave for a quest? Slaves in the captured list rather than the slave list are not visible to quests.
Edit: While it will not fix your problem, if you are playing on version 1.0d without conflicting mods, then the Bugfix mod can address many other issues.