Have you posted anywhere about this? @NatJ and
@Amer are a couple of the folks here that maybe could help you.
9 out of 20 viable. No quitters, the rest look unfertilized. The young rooster has only been here a little over a month, but there might be another reason. I've been reading about comb-genetics on threads here on BYC and apparently homozygote rosecomb could give lower rooster fertility.

I might have to rethink my plan for an all-rosecombed flock.
That is pretty much what I've read, too, about homozygous rose comb roosters being less fertile than others. I've read that the sperm does not live as long after mating (so the rooster needs to mate with each hen more often), and I've read that they tend to mate less often. I have no personal experience to back up any of this.
The obvious workaround is to have fewer hens per rooster, so it is easier for him to mate with them all frequently.
I've also read that there are two different rosecomb alleles, one with that effect on fertility and one with no problems. If you could breed a flock with the other rosecomb allele, you wouldn't have trouble with fertility-- but there is the trouble of getting the right allele in the first place, and recognizing when you have it.
A study that talks about the matter:
https://pmc.ncbi.nlm.nih.gov/articles/PMC3386170/
"The Rose-comb Mutation in Chickens Constitutes a Structural Rearrangement Causing Both Altered Comb Morphology and Defective Sperm Motility"
Published in 2012
(Not easy reading, but it is the most authoritative source I can easily find. It does contain a table of what breeds were found to have which rosecomb alleles. I don't know if any of them would be easy for you to get, or whether they would even be welcome in your breeding program because they might bring other traits you don't want.)
For your flock, as a practical first step, I would probably give the rooster more time before you make any drastic changes. He might improve with time and practice.
If that doesn't work, I might split the flock so he is with just a few hens, the ones you most want to hatch eggs from. If you want to hatch eggs from them all, maybe give him one group, get a batch of their eggs developing nicely in the incubator, and then put the next group of hens with the rooster. You could work your way through the whole flock in small clumps.
And most of that was probably stuff you already read elsewhere, or thought of for yourself before I got around to typing it
