Target display mode not working late 2012 models

Hi,

I have a late 2012 27" iMac running yosemite, and a late 2012 MacBook Pro running yosemite. They are connected via a thunderbolt to thunderbolt cable. The iMac has both a bluetooth keyboard and wired keyboard but neither will command it into Target display mode. I haven't tested a different cable, so that could be faulty but can anyone suggest a different reason for the issue?

Thanks.