Target display mode has suddenly stopped working

I have been using target display mode using a thunderbolt cable between my iMac 21.5 inch (late 2012) and Macbook Air 13 inch (early 2014) for around six months.

I had a few weeks off, and have come back to find that I can no longer enter target display mode! When I press Command-F2 on my keyboard connected to my iMac nothing happens. I've also tried Fn-Command-F2. I've tried different ports in case one them is damaged. But no avail.

Does anyone have any suggestions as to why target display mode would have stopped working so suddenly? I really like the feature of dual screens between my macbook and iMac and would really like it to come back!

Thanks.