It depends on the amount of character they have, first off, if they won't work or not. You need to beat out that character...
You send them to school for S and M / Bondage until their character attribute is zero. There are also other ways to lower the character attribute (buying a hypnotic ring for a girl) and even with the character at 0, it doesn't mean the girls will ALWAYS work, but will rarely not wish to work.
What I usually do is make sure the first girls I have work while a third goes to school for bondage (to rid of character) or another class to refine their skills. The higher the girls rank, the more school costs, so you ought to get this stuff done before you rank them up to any level over "Almost Beggar"