Picky eaters

I have 2 male ferrets. One 2 year old (Koda) and a 3 year old (Dash). These boys are SO picky when it comes to eating, they only eat Marshalls Premium. I have tried different treats, all ferret friendly but they want nothing to do with it. I've also tried giving them some cooked chicken. They snub their noses up at it. I'm concerned that they don't get enough variety in their diet. Is that just a me concern or can it cause dietary/health problems for them later in life? Do you have any suggestions on different foods/treats to offer them? Thank you in advance.