Don't shoot at the gobbler if it is near a hen. Hens are protected. You really don't want to look the game warden in the eye while trying to explain why you shot at protected bird. Keep calling and sooner or later you will be able to get within 75 yards or less of a gobbler. Hide yourself as well as you can and start calling.
Turkeys have an amazing variety of calls and they all mean something different.
The call that usually works best is the lonesome hen call. If you make it sound real, the gobbler will sound off immediately. How often you should call after the gobbler sounds interested is determined by how he acts. If he sounds loudly and quickly, call often, every two minutes or so. Otherwise only call every ten minutes or so.