My ipad air home button works but won't bring up lock screen - any ideas ?

What you're describing is what's intended to happen.

The home button does not re-prompt for the passcode. Not once the passcode has been successfully entered.

If there's more than one page of apps present and the current display is not the first page of apps, the home button does bring the display back to the first page of apps.

Or if you're running an app, pressing the home button brings you out of the app and back to the grid of icons.

iPad Air first generation does no have Touch ID in the home button, nor Face ID support, which means entering the passcode each time that's required. But the home button should only show the passcode prompt when the iPad has not yet been unlocked within the current session.

https://www.lifewire.com/what-is-ipad-home-button-1994353

So the home button lights the screen, but doesn’t bring up the passcode prompt? What happens?

Does the sleep/wake button on the top change how the display acts?

Maybe try restarting the iPad? Restart your iPad - Apple Support