There are many instances where Alexa may not respond to your commands.
Firstly, make sure your device is paired to your device. You can check this on your Alexa app enabled skills.
Make sure your Alexa device is not on mute.
When you are commanding Alexa with voice, make sure a blue ring is seen in the top left corner of the TV screen. This means that Alexa is listening for a command for your device.