how did you do the nuc? you need to put the queen in the nuc with just one frame of brood, a drawn frame or two to lay in, frame of stores and foundationthen shake in a good few frames of bees from the brood box. Do not plug the entrance with grass and do not move the nuc to a site elsewhere, but rather leave it near the mother hive. That way, all the flying bees will quickly return to the mother hive, so you only have nurse bees left in the nuc thus you have lost all your flying (swarm instigator) bees.