Hello ... I believe once the problems with sync start the system is in a state of "Lost in the middle", and changes are no longer passed on. This happened to me. You can durably solve this issue by bringing all computers to exactly to the state in which the iCloud copy of your bookmarks is, and syncing starts working again flawlessly: Across my Macs, my iPhones and my iPads - everything worked.
1) Export your bookmarks: In Safari go to File -> Export
2) Switch off iCloud synchronization for Bookmarks in "System Preferences" -> iCloud.
3) Exit Safari.
4) Delete current local copy of Bookmarks: Go to Finder -> Go, press alt-key to show the hidden menu items under "Go", click on Library. In Library find Safari and drag the file "Bookmarks.plist" to a safe location.
5) Switch on iCloud synchronization for Bookmarks.
You will see the system doesn't propose to "merge" anymore (because there are no local bookmarks anymore to merge with). After the initial synchronization your bookmarks are completely in sync and keep in sync for whatever changes you do. You may need to check whether the iCloud copy which was synced to your local user included all Bookmarks and you can add the missing ones by using the file you exported in step 1.
I had to repeat this procedure on every Mac but then I had never again any problems.
On the iPhone, iPad things are much easier. You just switch off sync, choose to "Delete Bookmarks on iPad/iPhone" and switch it on again. Presto.
V.