If the parent stock are both heterozygous for rose comb (only one copy each of the R comb gene) then 25% of the offspring won't receive it at all and will have a single comb. 50% of the offspring will be heterozygous as well and only 25% will luck out and be homozygous.
To add to that problem if one parent is homozygous and one is heterozygous, then all offspring will have rose combs but 50% of these will also be only heterozygous for the trait. This is why hatchery quality birds lose traits. It's extremely easy to have presentation of a dominant trait like rose comb but not actually consistently pass it on.