Contacts will not appear on my iMac

My Contacts and Smart Groups have not been showing up on my iMac (version 10.13.6) for some months (though the Contacts app opens), yet they are fine on my MacBook Pro (also version 10.13.6), iPad, and iPhone. All I get is suggestions from "Siri found in Accounts". I've confirmed that all the Contacts boxes are checked in iCloud and Mail preferences and that my account is correctly listed. I've started and restarted the Contacts app as well as the iMac numerous times. I haven't updated my OS yet because one of the softwares I use often won't be supported. Any help would be appreciated!