Go to Settings → Contacts → Accounts
Usually one of the accounts listed should have contacts turned on. For many people, it is either their iCloud account or a Google account. Try not to have more than one on. It creates confusion.
If contacts are syncing on one device only, check settings → contacts → accounts on that one and see which account has Contacts enabled for syncing. Then turn that account on for the other devices.
For Apple/iCloud, after clicking the account, hit iCloud to turn sync on and off for things.
The same for above can be done for calendars too.
Another way to see what Contacts are Syncing
Go to the Contacts app on your iOS device. Tap Groups in the top left. That should show which cloud accounts are syncing with contacts