I've never had a rooster past 15 or 16 weeks old so I'm not sure what mating usually looks like. The mounting is something that only happens for a minute or so and its only with one of my rir pullets (19 weeks but not yet laying). I have 9 other hens in my flock and if this one is a Rooster...
I recently bought 3 "hens" from a swap meet and I'm suspicious that one is a roo. They were being sold as 3 hens and a rooster but I wasn't interested in the rooster so the gentleman took him out of the crate. He told me that the were around 20 weeks old and that they would likely start laying...
It may just be that they aren't mature enough to lay yet. Do you know their breed? 20 weeks is usually the earliest any hen will lay. Some take much longer!