Q: Target Display Mode NOT WORKING

I have a 21.5' iMac (Mid 2011) and a 27' iMac (late 2012) that I would love be able to use Target Display Mode. Unfortunately I have tried everything I could think of to get it to work. Here is a list of things that I have tried but have been unsuccessful at enable TDM:

1.) Completely restored Mac OS X 10.9.5 on the 21.5' iMac. Setting this machine up as new since it will be my external display.

2.) No firmware password enabled

2.) Reset PRAM on both machines

3.) Connected an Apple Keyboard w/ numeric keypad to the 21.5' iMac.

4.) Pressed CMD+F2 to enable TDM (no luck)

5.) Tried to enable 27' iMac as external display

Repeated same process with an 13in Macbook Air (Mid 2011) and got the same result. TDM will not work.

So I did some extra troubleshooting to try to target why TDM won't work.

I put my 21.5' iMac in target disk mode and connected it via thunderbolt with my 27' iMac but it doesn't see the 21.5' iMac while in target disk mode. I also tried the same process with my 13' MBA and I don't see the 21.5' iMac while in target disk mode.

Does anyone have a clue to why this isn't working?