This time of the year, when some gobblers get "henned up", they become reluctant to leave any hens they are with to go to a call of a hen they cannot see. Some gobblers are reluctant to go to a hen calling because each time they have done that previously, they have gotten the snot beat out of them by a more dominant gobbler. I doubt that your calling is the root of the problem. I have heard (and made) some pretty awful calls while a gobbler was coming in. Unfortunately, the best teacher for learning to call is experience. The main thing with any type of call you are using is cadence. Make sure the yelps are quick and crisp, with 5 to 9 yelps. Make a few calls and stop, listening for a gobble. If a gobbler responds, don't call back. If he is interested, he will listen for a return yelp, and if he does not hear one will gobble again. When he gobbles without being prompted, hit him with a four or five note yelp. Get him gobbling to you, rather than you yelping to him. If you call too much, he will assume you are a hot hen and he will wait for you to come to him. The trick is to get him so hot and frustrated, he will come seeking the stubborn hen. This is the approach that I have used for years and it works more often than not. But nothing works 100% of the time.
A mouth diaphragm call is the most difficult call to use. A box call is the easiest, followed by a slate (pot) call. After a gobbler gets fired up, start toning down your yelps. In other words, call more softly and less often. Learn how to run the peg on the slate to sound like a contented purr. As the gobbler gets closer, yelp only 3 or 4 times very softly. Purr a bit. Use one hand to scratch in the leaves, like a turkey scratching for acorns. Also, the time of day will often make a difference. Off the roost early in the morning, gobblers are concerned about getting their harem together. If possible, get within about a hundred yards of where they are roosting. Don't start calling until you hear the hens start some soft tree yelping. Mimic the hens you are hearing. If you can sound like them, you will sometimes be able to call the hens in. The gobbler won't be far behind. Bringing in hens is more difficult, as the "boss" hen will often not want any competition and end up leading the gobbler away from this unfamiliar hen. But if you can manage to get the boss pissed off, she just may come right in to kick your butt.
I have been hunting turkeys for 25 years, and I am still learning. No two gobblers are alike, and no gobbler is the same two days in a row. Don't get discouraged, but keep after it. The only way to get better at it is to keep trying and learning. I hope I am not being presumptuous with my post, and if so, I apologize. I am only trying to provide information that you may or may not know in an effort to help out a fellow turkey hunter. Good luck to you and keep after it.