They may not feel secure. Clowns are more likely to spawn if they are the only tank inhabitants. She may be forming eggs, and then reabsorbing them.
You can try covering the tank on 3 sides to make them feel safer, or increasing the lights to 15 hours a day. That has been known to induce spawning.
But really all forms of stress need to be removed from their environment.
Clowns are very good at telling time btw. they need to be fed the same time everyday; otherwise they get stressed
This may not be good for your tank, but it helps them spawn