What are the Settings for iCloud on your iPhone? Is iCloud Photo Library enabled on the iPhone? If the iPhone is running iOS 11, iCloud Photo Library may have become enabled on the iPhone, and then not all photos will appear in the import panel on your Mac, because the originals may only be in iCloud and not on the iPhone. In that case connect the iPhone to Wi-Fi and view all photos enlarged to force the download from iCloud, before you connect it to the USB port to download the photos.
If you just upgraded to High Sierra, make sure you update iTunes to the newest version. Then run iTunes at least once with the iPhone connected. This will allow you to confirm on the iPhone that it can trust the Mac.