Have you enabled the Kasa skill for Alexa? If you've never enabled the Kasa skill, you need it now.
It used to be that you didn't need to enable skills for smart switches like Wemo or Kasa. But that was when Amazon didn't even have skills, so support for some of the more popular smart switches were built-in natively. Since skills came around, many of the older smart switches still worked, but with some glitches here and there.