Target Display Mode Not Working: From 2015 MacBook Air to 2009 27" iMac

2009 27" iMac - 2015 MacBook Air, both running macOS High Sierra v10.13.6, connected by mini display port cable purchased from Apple.

On pressing F2 on the iMac, both screens goes black and then the iMac displays the MBA's wallpaper but it does not display and programs working on top of the wallpaper or the dock. The only thing I could get it to mirror was when I opened System Preferences, Display (it mirrored the 'Display' tab only) or System Preferences, Desktop and Screensaver (it mirrored the 'Desktop' tab only. When I changed the wallpaper on the MBA it did not change on the iMac, even after switching TDM on and off pressing CMD F2.

Any ideas welcome. Thanks