Search for the Turkey School posts farther down in this section. Those will give you a good idea of what calls to use, when, and how often.
Early season birds are defending their breeding territories until the hens are ready. Once the hens are ready to breed, that's all they are interested in. As the breeding cycle is ending a gobbler will travel looking for a hen, ones the breeding is over gobblers prefer to be alone to eat and rest up from all the fighting and running they do.
Your calling tatics have to change along with your setups as to what part of the breeding cycle the birds are in. What worked early season may not work at all later in the season. Hard to work birds are often sub-dominant birds, that have gotten put in their place by the boss gobbler, so they may be shy at gobbling or choose not to gobble to keep from gettin in trouble again, they come in silent or circle and not come in at all. That's when knowing what calls to use, volume and how often you call is most important. Learning to take a birds temp is important, you'll learn from how a bird answers your calls as what state of mind he's in. If he's a hot bird, he'll blast an excited gobble back at you as soon as your done calling or he'll cut your calling off. If he's a sub-bird he may gobble back softly and it may take a few minutes for him to answer you.