Adresbook over iCloud between 4 Apple products (MacBookPro, iMac, iPhone3, iPad2)

MacBookPro (MBP), iMac: OS 10.7.5

iPad2: OS 6.1.3, iPhone3 iOS 6.0.1

iCloud activated on all 4 products, incl. checked "documents & data" and obviously "adres book/contacts"

Very recent changes made in adresbook on MBP appear in iCloud immediately, even test changes made in MBP appear after refresh of browser.

1. The iMac "refuses" to accept these changes in its adresbook, although its connected to iCloud, comparable to the MBP

2. both iPhone and iPad equally refuse accepting the new adresbook in iCloud

I recently "forced" them to accept the latest version by exporting the current adres book on an iFlash drive and then importing it into the adresbook on the relevant product, but that's not the idea behind iCloud!!

Where's my hang-up, what am I missing? Thanks