try seperating, so they cannot see or hear each other or other doves, then re introduce after a week. the female maybe more receptive to lay in a nest with over hang to cover top of nest, and lip or nesting material enough so eggs wont roll out or other added nesting material. like a sideways box big enough for both to fit in comfy, and leave one or two flight feathers in cage, one in nest. my flying flight hens are picky and wait to lay till the nest and courtship are perfect, even after month of mating and courting.
never unfortunately had diamond doves, though my ringnecks took bit to get going and lay, incubate, and rear squeakers, but then once they had the idea and apperantly hang of it, they were some baby raising machines, so watch out you dont get over run quick, and let them take their own timne and make sure theyre in tip top shae and mature enough even. maybe take nest and all out and wait six months at least to a year.