I can use airplay on iMac but not iPad or iPhones

My iMac streams music to my Apple TV units and I have the airplay icon appear. I can happily stream music to different speakers from my iMac.

I am using Pandora to play music on my iMac and would like to stream it to different speakers. I thought I could do this using either my iPhones (4S & 5) and iPad2 but no such luck. It would appear that I cannot stream to the AppleTVs as they are first generation but they happily work with the iMac?!?

Am I missing something or is there a way I can achieve streaming from these devices using Airplay?

Background: The wifi is fine, all devices are using the same system and are connected using a router. It works well for the iMac using itunes but alas Pandora is not available on itunes.

Any suggestions?