iPhone 6 will not connect to iMac

Hi everyone,

My iPhone 6 will not connect to my iMac via USB. I am running up to date IOS on both iMac and iPhone. It will not charge or appear as a device.

The phone charges when I plug into the wall.

It connect's to my girlfriends Macbook Air.

The iMac is for sure a trusted device.

I have tried two different cables.

I have tried each and every USB slot.

I have reset the SMC on my iMac and NVRAM - did not work.

I have cleaned out the iPhone charging port to the best I can.

When I first started having the problem, the iPhone connected for a few minutes and then disconnected without me doing anything and after that it will not connect again.

Anyone have any ideas? I would really appreciate it! Many thanks!