Target/Extended Display Mode Problem on iMacs

I have a late 2012 27" iMac (High Sierra 10.13.6) that I wish to use as a secondary monitor for my new 27" iMac. I have purchased the Thunderbolt 2 cable and the Thunderbolt 3 to Thunderbolt 2 adaptor. I have followed various tutorials to place my secondary monitor in Target Display Mode. When I do, both screens blink black for a second and screen on the 2012 computer stays black. However when entering the Display System Preferences on the new computer, I do have the option to arrange both displays, but just to repeat, I cannot see anything on that secondary display.

Can anyone help me? I don't want my screens to mirror. I want to use my old iMac as an extended display and be able to drag various windows between the two screens.

Thanks!

Kelly