Are they male/female pair. Most fish will "fight" before they breed. I've had Convicts where the female would nip the fins and harass the male but they would always breed soon after this.
They fight to see if the other is fit enough to breed. Where did you purchase your pair? Have you seen their vents drop or are they cleaning an area with their mouths? I've had very aggressive pairs and sometimes 2 males/2 females.