Why won't he come anymore?

Hi everyone! It's me again with another problem. It's NiNi, he used to come whenever we call him but lately when we call him, all he'll do is look at us and either just stand there or lay down. He won't come to us for anything except when we give him a treat. I don't understand it, he had been coming to us since we got him but now, nothing. What could be wrong?

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