Do you feed any greens or hay or alfalfa or anything? Nutrition is one thing but one also needs to keep in mind that bacterial flora and fauna in the digestive tract are also incredibly important. On it's own the rabbit would live primarily on greens and grasses, I had one escape in 2012 when the cage was attacked by a dog and she is till out hopping around the farm happy as a lark. It has been interesting watching her eat over the years since she escaped, and without any pellet feed or grain or anything she is healthier than she ever was in captivity.
You might a slow introduction to some greens and hay, rabbits love alfalfa and you might try some different grains in small quantity. For fattening I have found with most everything corn or dried peas are good, they work awesome on rabbits, chickens, goats, hogs etc.
P.S.. I have never raised rabbits for show, but we have had up to 100 to 150 at a time raised for meat. One thing I noticed right off with our rabbits is that they really didn't care much for pellets. I had small tuna fish cans as pellet dishes in their individual cages and they eat through a tuna fish can full in a week generally or some would never actually eat the pellets at all.
The main feed they ate was the weeds and grass that we would pick and feed into the cages each day, with the fresh greens they really wouldn't hardly touch the pellets. They also seemed to prefer leafy hay and alfalfa over the pellets as well. As for grain it was about like the pellets I could get them to eat "some" but like the pellets not a lot. Rabbits seem to have a great natural instinct for what is best for them to eat. I don't know if you are constrained to any feeding rules on these rabbits but from my experience if you want them to eat more, fresh greens are the way to go. I would stay away from most store bought vegetables and also be wary of any greens that might have been sprayed with fertilizers, weed killers or insecticides. Not a problem for me as I own my farm but in a town or city setting that would be a concern.