Many cats just won't keep them on. More proof that they just have their own minds!
My daughter puts a collar on her outside cat and she's always taking it off. I don't know if there is an answer to your problem. Mine are microchipped and are indoor cats. I don't have small children to let them out and I live alone. So It's unlikely they'll get out. But others who have said not everyone looks for a microchip, is right. It's a bone of conention with me that even a shelter may not look for that valuable chip.