In a web browser, visit iCloud.com, log in with your Apple ID, go to the Photos area of the website.
If you cannot see those recent photos, then you have an upload problem on your iPhone.
If you can see those recent photos, then you have a download problem on your iMac.
One possible reason for a download problem on a Mac is (paradoxically) an upload problem; uploads must complete before downloads are attempted.
What version of iOS is running on your iPhone?
What version of macOS is running on your iMac?