Problems syncing contacts between new iMac, iPhone, iPad using iTunes and USB cable

I have just acquired new iMac and have always been using iTunes to sync contacts and calendar from my iPhone8 and iPadPro via usb cable with previous MacBookPro. I have synced my iPhone contacts to my new iMac with no problems but if I then add an additional contact on my iPhone and resync using iTunes it will not show on my iMac contact list BUT if I then connect my iPad to my iMac and sync using iTunes the new contact although not shown on my iMac is actually shown on my iPad. If I do any alteration on my iMac contacts they are also not synced via iTunes to either iPhone or iPad. All iMac, iPhone and iPad have the latest software upgrades.