Messages not syncing across devices

Colpolyp Said:

"Messages not syncing across devices: No, I'm not talking about SMS messages, I am talking about Apple Messages on which syncing fails, including "Text Messages Forwarding" which is also an iMessage feature."

Restore to Factory Settings:

If you are so concerned, then reset your iPhone to factory settings.That seems it would fix this. Go here: What to do Before you Sell, Give Away, or Trade in your iPhone, iPad, or iPod touch - Apple Support

Backing Up Items:

Note that all downloads would be lost. So, make a note of Apps that were downloaded. Also, enable items in iCloud, so that all is backed up. As an example, here is how to backup Contacts...

I. Enable Contacts in iCloud Settings:

Is Contacts enabled in iCloud Settings? If not, then enable it, and see if it pops up. If so, then it is gone amongst all devices associated to your Apple ID. So, to enable it:

A. Enabling on an iPhone:

Enable Contacts in iCloud:

1. Go to: Settings
2. Tap: your name (top item)
3. Tap: iCloud
4. Tap: Contacts (so that it is green)

and...

B. Enable on a Mac:

Is Contacts enabled in Apple ID on your Mac? If not, enable it.

1. Go to: System Preferences
2. Click: Apple ID
3. Enable: Contacts

Hi there Colpolyp,

Welcome to Apple Support Communities. Do you have Messages enabled in iCloud on both devices? Doing so ensures that the messages sync properly across all devices using the same Apple ID and password for Messages. The steps to check, and enable if needed are found here:

Use Messages in iCloud

Feel free to reach back out if you have any questions.

Best regards!

Colpolyp Said:

"Messages not syncing across devices: Both iMessages and SMS messages are not syncing across three iOS devices (15.5) and two macOS (Monterey and Catalina) devices."

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Caches Getting in the Way:

"Other" being the Culprit:

The storage named "Other" is the grey bar. So, to clear that, use my User Tip:  How to Clear "Other" in your iPhone's Storage - Use Backup - User Tip

Colpolyp Said:

"Messages not syncing across devices: Thanks, but none of my devices have any data in the ‘Other’ category. (See my aforementioned post). Is there anything else I can do?”

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You are welcome.

Whom to Contact:

Contact your Carrier, as they are the one providing you with the service of Massaging.

Report back what they have to say on all of this.