photos not syncing with iMac

Also, make sure that both devices have a chance to connect to the Internet.

On the iPhone, you might have things set up so that Photos is allowed to use Wi-Fi, but not cellular data. If so, then you would need to connect the iPhone to WI-Fi Internet service every now and then to let it synchronize its Camera Roll with iCloud Photos. (If you have Wi-Fi Internet service at home, then the iPhone probably gets that chance any time that you have it at home.)

You likely missed something in your iCloud setup. Please check both devices and ensure you have adequate iCloud storage.

iCloud - Learn how to set up iCloud

Failing that, just give AppleCare a phone call. Click the following link for the phone number. AppleCare Contact Info