My two younger and smaller chickens have some issue with thier combs. They are definitely bloody possibly from the other girls picking on them. I want to confirm that it is not fowl pox which is something I gave heard about.
If it's only on the 2 younger and smaller birds then I'd say it's most likely just the others picking on them... separate til healed or get some blue kote or similar stuff to put on it til it heals...
Pecking order issues can get bloody, hence the pecking part of the order, lol...
Also, fowl pox would not just be on the comb, it would be around the nares, eyes, beak and wattles as well...