Why won't my oscars grow?

I have 2 oscars in a 60 gal. tank with a 4 inch pleco. How come my oscars do not seem to be growing very much? They are about 6 in. and 5 in. I have them on a diet of pellets, frozen bloodworms and brine shrimp, and I recently added freeze dried krill and bloodworms. Is there something that I can do to encourage growth?