How can I sync old messages between iDevices? Recently Messages stopped working on my Mac. Messages sent by my iPad and iPhone were synced to my Mac, but sending failed. After trying about twenty-eleven possible solutions from here, apple, and elsewhere on the internet, I rebooted my Mac.  This doesn't happen often.  The problem went away, but so did the last 2 months worth of messages.

Is there a way to sync past messages between multiple devices?

Iphone/ipad are on iOS 10, Mac is on Yosemite.
Generally speaking, the answer is no. Messages are saved on Apple's servers for up to 30 days, but only if the message has not been confirmed as received by the device. Even if it was still on the server, there's no way to get the server to resend it to your device. Nor is there a way to sync messages between devices.

You could save Messages history off your iPhone with something like iExplorer, but there's no way to integrate that into the Messages app on your Mac.

As far as them disappearing when you rebooted, under Messages > Preferences ensure "Save history when conversations are closed" is enabled.