Existing text messages not showing up in Messages app on new MacBook?
Messages don't backdate &amp; sync.

Each keeps its own record of messages it sent &amp; received &amp; syncs to other listed devices it can communicate with at the time.  

If, for instance you receive messages at your Mac whilst your phone is out of network coverage, they will not appear later on the phone; The messages will be considered to be delivered when any registered device accepts it, &amp; it will not be retransmitted to the other device later.