Unable to activate Siri with just my voice on my iPad Air

You are going to want to make sure that you have Siri settings that you want to use turned on. For instance if you want Siri to respond to “Hey Siri” you’re going to have to turn this on. It may have gotten turned off, Settings > Siri & Search is where you’re going to find a majority of these settings. If you want your iPad to respond even when the screen is covered or facing down, then you can go to Settings > Accessibility > Siri and adjust the settings in there that you want such as “Always Listen for Hey Siri” in which it will respond no matter what the situation is. There are also other settings such as if you are in close proximity to your other Apple devices and allowing them to distinguish which one should respond so you don’t get all of them responding at the same time because they can all hear you.