it's true..the discus have individual traits..but the growth rate is note soley determined by this.
if you have several discus you may find some are more dominant and will get to the food faster and eat faster..this inturn rduces the available food to the less dominant fish in the ladder.
these will get less food and will usuallly be slower at growth.
but you do need to get your fish to eat.
try beefheart, mine don;t particulalry care for it, try frozen -thawed bloodworms. brine shrimp etc.. mix your hikari(mine don't like that either) with the bloodworms..the fish wil eventually associate the flake with the bloodworms and you can then cut back on the bloodworms.