Why aren't my contacts syncing across my Apple devices under same Apple ID?

Lilleth wrote:

I have several devices, including more than one iPhone and iPad, and they are all on the same Apple ID, but I find that my contacts are not synching. Several months ago what was in one device would appear on the other within minutes, now it just doesn’t do it. I’ve tried logging out of my Apple ID on both devices and re-logging on, and merging everything that is on iCloud, but still no synching. There is a discrepancy of almost 100 contacts. Please help.

[Re-Titled by Moderator]

For sync issues sign out of your AppleID and back in again

ref: Log out of Apple ID