Do you see the caller name when you receive the call? In that case, the lookup is working, but maybe the 3CX Web Client is not able to popup the contact record.
So, first check that the contact lookup works. Enable verbose logs and check the 3cxSystemService.log file.
If you see the caller name, then check that the 3CX Web Client > Settings > Integration, is configured to automatically open the contact record. If that's the case, please also check that the browser is not blocking the popup.
Finally, if you're using 3CX v16 Update 4 and the new Chrome Extension, the popup will not happen. This is a known issue that will be fixed in Update 5.