Command + F2 does not turn iMac into a screen

hi, i am trying to use my 27" iMac as a screen to my Macbook Pro Retina.

I have the thunderbolt cables plugged in, but pressing Command + F2 does not work. Yes i am pressing these buttons on the iMac.

The first time i tried it a few days ago, the iMac did turn into a screen, but after a while it fell asleep and the connection was lost. I have not been able to restore it again, which is annoying.

Another guy at work has the same issue and he has found it randomly works and sometimes doesn't.

Does anyone know why? feel like the $50 spent on the cable is a massive waste of money if its not going to work. Wish apple would fix this.

Any help is appreciated!

i have also looked at http://support.apple.com/kb/HT3924 and it doesnt have anything i dont already know