Only some messages being synced between iphone and Mac

Hi nickkoo,

We'll be glad to help out with syncing your messages. One potential cause for them not all appearing, is if you do not have text message forwarding set up on all devices in use.

To set this up, follow Forward SMS/MMS text messages from your iPhone to your Mac or iPad - Apple Support

"Automatically forward texts to your other devices

1. Make sure that you're signed in to iMessage with the same Apple ID on each device:

• iPhone or iPad: Check the Apple ID shown in Settings > Messages > Send & Receive.
• Mac: Open the Messages app, choose Messages > Settings (or Preferences), then click iMessage. Check the Apple ID shown.

1. On your iPhone, go to Settings > Messages > Text Message Forwarding.
2. If you don't see Text Message Forwarding, go to Settings > Messages. Turn off iMessage, then turn it back on. Tap Send & Receive, tap Use Your Apple ID for iMessage, then sign in with the same Apple ID used on your other devices.
3. Choose which devices can send and receive text messages from your iPhone:
4. If you choose a device that isn't using two-factor authentication for your Apple ID, a verification code appears on each of your other devices: enter that code on your iPhone.

As long as your iPhone is turned on and connected to Wi-Fi or a cellular network, new SMS/MMS texts can be sent and received on the devices that you added."

Setting up Messages in iCloud will also ensure that all messages are synced across all devices. That can be set up by following Set up iCloud for Messages on all your devices - Apple Support

That should solve this for you.

Cheers.