@Butrflynet,
Thanks, butrflynet. The homminy I'm using is dried, from a Mexican market. The kernels are enlarged, apparantly from being soaked in lime water, but they have a tough skin on them. Canned homminy lacks the skin and is more tender.
I thought about adding salt, but, as you said, that would only toughen the skin. I guess the answer is to either buy canned homminy or continue as I have been doing. The homminy turns out chewy but adequate for my purposes--in stir-fry.