I saw this behavior in speech therapy with my second. He would refuse to vocalize to his mom because he knew she would eventually cave and give him what he want

I saw this behavior in speech therapy with my second. He would refuse to vocalize to his mom because he knew she would eventually cave and give him what he wanted, but when the speech therapist withheld a toy until he vocalized, he responded very quickly - likely because he was unsure whether she would cave as mommy does. This helped us realize that, in this case, we really were the problem. We needed to, in some cases, be more firm. At first this resulted in a lot of tantrums, but once the child's expectations changed then he behaved with us similarly to with his speech therapist.