Hi David,
Welcome to TONMO.com!
First of all, some questions - how big is your bimac? If a bimac is laying eggs, it's going to be fairly large.
If your bimac is smaller, perhaps it's finding food in the live rock, such as amphipods. Also, are you sure it's a bimac and not a nocturnal species?
What are you offering in the way of food? Have you tried live crabs or shrimp?
Tell us a bit more, so we'll be able to give more specific answers.
Nancy