Is the problem that they refuse to breed or that they breed but do not have kits as a result?
Refusal to breed may be due to the time of year. Cold temperatures in particular may cause a doe to refuse. Try choosing a sunny day with above average temperatures to counter this.
Vitamins A & E are essential for successful breeding. I know they are in the pellets, but sometimes due to the processing techniques and/or storage conditions, by the time they get to the rabbit these vitamins may be lost. Adding 2 tsp. of wheat germ or 5 - 6 drops of wheat germ oil daily to their feed should supply their need for Vitamin E. For Vitamin A, try supplementing with dark green leafy foods such as parsley, carrot tops or dandelion greens. It seems to take about two weeks for the vitamins to help, and it will, of course, only work if a vitamin deficiency happens to be the problem.
If the rabbits seem to breed (buck mounts doe and falls off) but no kits result, it could be that the buck is not fertile. The chances of this increase if it is happening with a number of different does. Has your buck fathered any kits at all? If you have a second buck, try using him instead and see if that helps.
If some does catch and some don't, then you need to look at the individual does and try to figure out why. Do you know the ages of your breeders? It is often hard to tell a rabbit's age unless you acquire it as a youngster that is still growing. It is not unknown for people to sell older rabbits near the end of their breeding years to unsuspecting buyers, passing them off as younger stock. Sad but true.
There are other reasons why you may be having difficulties. Knowing more about your rabbits and how you feed and house them will help us to help you figure it out.