I suspect the feedback loop may not be working as expected. You may want to try the open loop circuit simulation first, break the loop and run the flyback at fixed duty cycle. This will help get identify the problem if any in power and feedback sense circuit. If this work, you can try closing the loop and see if simulation works. In lot of cases error amplifier gets saturated at one end of power rail and this causes circuit to stop function as there is no switching and hence no corrective action.