Theres been a few reports online of messages sync being a bit buggy if you don't set it up exactly right. Heres a few solutions that should get everything syncing nicely for you:
Firstly, you need to have all devices set up to the same Apple ID
iOS Devices (iPhone, iPod Touch, iPad):
- Navigate to Settings > Messages.
- Make sure iMessage is turned on.
- Tap on Receive At.
- Check your Apple ID. Make note of this Apple ID.
- Tap on Caller ID.
- Use your email address. If you use your phone number (assuming you’re on an iPhone), then the non phone devices (including the Mac) may not (and probably won’t) see the messages.
- Navigate to the Messages app.
- Go to Preferences > Accounts.
- Add the same Apple ID as you used on the iOS devices.
In a nutshell: everything needs to be consistent. Use the same Apple ID everywhere and an email address as a caller id for best results.
If things are still not syncing, try the following.
- Delete all your previous conversations: Some people have reported that they had to delete all of their conversations before things would sync up.
- Make sure you have your phone number added to your Apple ID account online.