Obesity, lack of light, buck sterility etc. are all reasons for failure to breed. One thing that is sometimes overlooked, however, is nutrition. Even if you are feeding a "complete", good quality pellet, it is quite possible for the diet to be deficient in Vitamin A and Vitamin E, both of which are essential for successful breeding. Pellets contain these vitamins at the point of manufacture, but they can be lost through less-than-optimal or prolonged storage.
Try supplementing your rabbits' diet with some fresh, dark leafy greens. Dandelions leaves and parsley are both excellent and are often available in the produce section of the grocery store. These will supply Vitamin A. If your rabbits are not accustomed to greens, introduce them slowly... just a leaf or two to start and gradually increasing them.
Vitamin E can be added by using wheat germ, wheat germ oil, sunflower seeds etc. You can google to find other sources of Vitamin E rich foods.
Sometimes increasing these vitamins for a few weeks will make a tremendous difference in fertility. Of course, it will only work if the problem is indeed nutritional.