I'd say very poor genetic's but diet can also affect wen growth. I have some comet/orando hybrids in my one pond that have a lot better wen's. But they also took 2 or 3 years, I can't remember exactly anymore as they are over 10 years old, before they began developing wen's. Mind you they are not like the big full wen's of pure orando's but they are decent enough for hybrid's.
Goldfish need far more green in their diets than protein. Flakes are the worst diet possible for goldfish as they are poor in nutrition. I always use a good quality sinking pellet.
DL-methionine is the amino acid responsible for large wen growth. This is prevalent in greens such as garlic, spinach, peas, and spirulina, and kelp. I feed my goldies a lot of veggies and some fruits. Actually they are the bulk of their diet. Perhaps if you change your feeding including more veggies and get a good sinking pellet like the Hikari Lionhead sinking pellets you might see more development. It's worth a shot IMO