Can't modify viseme mouth pose mapping

I'm very new to Animate and I'm trying to make a cartoon chicken talk with four different visemes (including the neutral). I've made the different visemes on the same graphic symbol and placed them on different layers. After I go back to the chicken and place the mouth onto the chicken, I press the lipsync button. But it doesn't let me set up different visemes when I click on the different pronunciations? Can someone help please 🙂