First of all, are you seperating the sexes at any time? Females do not like to be constantly courted by males and need either a large area to get away from them or they need to kept apart except during breeding.
Also, are you seeing actual mating or just courtship? There is no mating unless the females allow it. Males will court and mount all the time but that is not 'breeding'. I have found that keeping the females by themselves for most of the year results in much more actual copulation. I try and keep them apart until March or April then allow the males access to the females off and on through the summer. During the summer the females are given a break for up to a few weeks at a time.
You will find that females, when ready to mate, will sometimes begin to mount one another as if they were males.
It also sounds like you have too many males in with the girls...this just causes the females to get more stressed and less likely to allow copulation. A non-receptive female will either always try and walk away from a male or will just 'sit on her tail' while the male wears himself out trying to mate.