Give them the silent treatment. Don't call at all. On birds that are uncallable, and yes, they do exist, try to pattern the hens they are with and ambush them. Have you tried hunting close to the roost in the afternoon? Set up close to their roost in the morning and see whhere they are going. Calling at these birds may be futile and hurt you more than help you. It will take a few days to figure out a pattern, but if you do figure it out, the hens may bring the gobbler right into your lap.