I'm trying to use iMac 21''(mid-2010, Mavericks) as a display for newly purchased Mac Mini (late 2014) and I'm unable to activate TargetDisplay Mode. The devices are connected using MiniDisplay Cable and pressing Command-F2 or Command-Fn-F2 does nothing (tried in when logged off as well), the screen doesn't even blink.
Unfortunately, the Mid-2010 iMac 21" doesn't support Target Display mode. Only the 27" iMac from that generation does, which is why you are getting no response when you press CMD+F2. The Apple website offers more information here: