Look on craigslist or post a wanted add for people who breed rats in your area. Get a few high quality females and raise them up. Hold back all females from the lines you want to continue to breed.

Along with their diet (I would suggest rodent block, but many have success with Doggy Bag dog food and a few other brands), feed table scraps (grains, chicken bones, cereals, veggie noodles, fruits, veggies, etc). This will help them keep a healthy weight as well as help them lactate when they have their pups.

Make sure they are not kept somewhere too warm or cold, this will slow production. The rats will be more focused on maintain their body temperature than reproducing.

Also, it could be that you have a lazy male, which is another reason to find a rat breeder in your area. I would pick up two-three males and at least six females. Hold back the male that breeds the best for you, and the females that produce the most and care for the pups the best. Usually after finding a group that breeds well, you can then focus on traits such as color and personality. Never hold back a female that eats her young, even on her first litter. I've never had a female who devoured her young not turn around and do it again.