Many people have found that they have to log out of the app and log back in to get it to work.
follow these numbered steps exactly in that specific order and see if it fixes the problem. Don’t worry that steps one and two are missing, they are for if you are using a virtual device. 
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use the Alexa app to disable the smartthings skill.
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log out of the Alexa app.
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log back into the Alexa app.
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re-enable the SmartThings skill.
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ask Alexa to discover new devices