I have an iPad Pro 9.7 Wifi & Cellular 128 GB. In iOS 10.3.3, I could call Siri with "Hey Siri" keyword even if iPad locked and Smart Cover closed. Now, I upgraded my device to iOS 11. The "Hey Siri" keyword runs only if iPad locked and Smart Cover is open. However; if iPad locked and Smart Cover closed, it doesn’t work. I've been using my iPad (in a Smart Cover) for "Hey Siri what is the degree in my room" for about a year now. It stopped responding only after updating to iOS 11. "Hey Siri" and "Allow Siri while locked" toggles are checked in Settings also.
This is a known issue with iOS 11. Currently there is no way to turn "Hey Siri" back on with the Smart Cover covering the iPad's screen.
Please provide feedback to Apple so the feature can be brought back in future releases of iOS 11.