Q: Getting imessage to sync between device

Have you double checked your settings?

1. You need iOS 8 or later on your iOS devices and OS X Yosemite or later on your Mac.
2. Sign in to iMessage with the same Apple ID on your iPhone, your other iOS devices, and your Mac.
3. On your iPhone, go to Settings > Messages > Send & Receive. Add a check to both your phone number and email address. Then go to Settings > Messages > Text Message Forwarding and enable the device or devices that you want to forward messages to.
4. Look for a code on the Mac, iPad, or iPod touch that you enabled. Then enter this verification code on your iPhone.

If everything checks out with your settings, try signing out of iMessages everywhere and start all over again. Remember that you need to check the email address and the iPhone number.

I Understand exactly what you are saying and I am suggesting starting all over again, because sometimes that is what it takes in order to get the code.  When I first set it up on my iPad, I had to make more than a few attempts before the code came over.

Excellent, I'm glad that it finally worked out. Like I said, I had to do it several times myself in order for it to work.