How to enable AirPlay on iMac?

My main computer is an iMac (27", Mid 2010, 2.93 GHz Intel Core i7, ATI Radeon HD 570 1 GB) connected to a 4TB external HD. I also have a MacBook Pro (Retina display, 2.4 GHz Intel Core i5, 8 GB 1600 MB). Both are running Mac OS 10.13.1.

I can airplay direct from my MacBook to my Apple TV, however I only rarely have the option to do so from my iMac.

WHY ???

If the iMac can airplay sometimes, why can't it do it always (like with the MacBook). I shouldn't have to drop a home video into iTunes to share it through my iTunes library on the Apple TV. This is counter intuitive.

Can anyone provide a solution?