iMac Pro not sharing desktop

I’ve added a layer to share the desktop. My setup: iMac Pro (5K monitor) + older Thunderbolt Cinema (2560x1440) Display.
The Thunderbolt shares fine, the iMac screen won’t. I’ve tried setting different resolutions on the iMac, on Manycam, etc., nada.
Any ideas from anyone?

New IMac here: 1) on both my Imac and on my Mac book pro, I Click: Apple. Systems preference. Sharing. Screen sharing. On my IMac task bar a new screen sharing icon appears and I click that… voila! then add into a manycam preset.
2) Connect Iphone to Imac: usb connect phone to Imac. Open QuickTime. Open New Movie Recording. Click down arrow by red dot. Select Iphone… viola! connect to a pre-set in manycam.
3) purchase mirroring360 for $11.99. connect your device to manycam. Youtube’s available.