It is common for young, juvenile animals to have some difficulty during the first time breeding.  Often we wait until a cat's second heat cycle to attempt breeding.  The male cat should also improve his ability by age 1.5-2 years.
There are 2 phases to the heat cycle-in the first discharge may be present and male cats become interested.  The second phase is where the female becomes responsive and allows the mating.  It may be too early in the heat cycle for a successful mating. 
I would have both cats examined by a veterinarian to make sure there are no medical reasons for the problem.