What is your current growth rate?
Making sure the pellets have a good balance of fat, fiber and protein will help them gain and maintain weight better. Generally, 3.0% fat, 20%< fiber and 18% protein is the ideal for meat rabbits. You can also give rolled oats and black oiled sunflower seeds for a little extra fat for your grow outs.
With that said, slow grow rates are commonly more genetic or environmental than a lack in nutrition. Fast growth rate is something that you most likely will need to breed for over time. Cage space is very important for growing rabbits, for my calls I have had the best result with now more than two grow outs in a 6 square foot cage (they reached weight at 14 weeks). Weather will also affect the growth rates rabbits who are born in very cold or very hot weather can often be stunted, my best growth rates were always with litters born in march/April and sep/nov/oct (the mild months in the PNW).