As long as it's eating I wouldn't worry about it not eating pellets. What you can do is to just start to feed it pellets and less of the other food, it should then start to eat the pellets after sometime. What type of pellets are you trying to feed it anyway?
Mix the frozen food and pellets together then feed. See if the fish will eat the pellets then. If so you can adjust the ratio of pellets to frozen food over time until it accepts pellets on their own. Not guaranteed 100% to work but it can.