You have to think of it this way: What is your dog's incentive to come? If you don't give him a reward of some sort, he has no incentive to come back to you.
I think your dog has inadvertently chosen not to come because he knows that playtime ends when you call him.
Rewards don't have to be food. Rewards can be praise and play. The reward your dog probably wants most would be to go back into the yard. So, call him to you, give him a treat, and *send him back to the yard*. Repeat this. Eventually, your dog will choose to come to you because you've rewarded it with play in the yard. Because you've rewarded the come with sending him back to the yard, playtime in the yard becomes devalued and his come will be much stronger.
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