iMessage has a feature that allows you start sending messages on one device, and ultimately finish on the other. If you have multiple iOS (iOS 5) devices (i.e. iPad, iPhone or iPod Touch), you can sync your conversations across all of them. All you need is to set each iOS device’s iMessage account to the same Apple ID. However, several users have reported that iMessage is not syncing across all devices. If your iMesssage is not syncing, try these fixes until you resolve the issue:
Ensure that you have a valid cellular or Wi-Fi data connection.
Make sure that iMessage is turned on (Settings > Messages.)
It is important to activate iMessage on all your iOS5 devices using the same Apple ID. If you already activated iMessage, tap Settings > Messages then turn off iMessage. From there, open by tapping Messages app on the home screen to sign in with your Apple ID to activate iMessage.
If you are still having problems; try this fix. This particular fix remedies syncing problems if you want to sync between iPhone and other iOS devices (iPad or iPod Touch). Complete this process on each iOS devices you have. Tap Settings > Messages > Receive At. Receiving At settings is your cell phone number on your iPhone by default. Add the same e-mail address on all devices as another “Receive At” location.
Tap Settings > Messages > Receive At > Caller ID and then tap on the Apple ID (same ID on all devices) as your caller ID.