If iCloud Photo Library is enabled on the iPhone, you have to view the photos enlarged on the iPhone to force a download from iCloud to the iPhone, before you connect it to the Mac. Otherwise the photos may not be available for download, because they are in iCloud and not in the Camera Roll. Which version of iOS is running on your iPhone?
If the iPhone has been upgraded to iOS 11, iCloud Photo Library may have become enabled automatically, without you noticing it. So check the settings on the iPhone.
Have you installed the latest version of iTunes on your Mac? If not, you may need to upgrade iTunes (iTunes - Download iTunes Now - Apple) and then launch iTunes, after you connect the iPhone. Once the iPhone appears in iTunes, you will see the usual "Do you trust this Mac" that comes up on the iPhone. Confirm this on the iPhone. After that, Photos should be able to show you the photos in the Camera Roll on your iPhone.