Target Display Mode between 2 iMacs

Hi - i have a iMac (Retina 4K, 21.5-inch, Late 2015) Intel Iris Pro Graphics 6200 1536 MB running Catalina wanting to use TDM on a iMac (21.5-inch, Mid2011) AMD Radeon HD 6750M 512 MB Mid 2011 running High Sierra

I have had it working twice with an apple genuine Thunderbolt Cable - but don't understand that it wont work each time i am selecting command f2 on 2011 iMac, the first time it worked was when i just plugged in the cable - then it wouldnt repeat. Lots of tries again, after restarts and shut downs, the last time i got it to work was after selecting Lock Screen on 2011 and command f2

any ideas...