iPod touch 5th Gen. not connecting to iTunes over wifi

Hi

My iPod touch has suddenly, in the last few days stoped being able to sync with the contents of my iMac over wifi. In the options part when a Device is conected to iTunes, there is a check box which I always have ticked. I have unticked this box and resynced, then reticked this box and resynced, but this has not worked. The only was to get music on to my iPod which I have on my iMac is to connect it the old fashion way with a cable.

The iPad mini has stayed connected. Both Devices are running iOS 8.2. I don't know what has done wrong