How do I load older iMessages?

I had to drop my current MacBook off at the Apple Store (the usual keyboard issues) so I got out my 13" Aluminum Late 2008 running El Capitan (the newest version it can run).

I opened the Messages app, and the newest messages displayed are whatever there were when I last used the MacBook (over a year ago).

If I send a test message to myself it works just fine and syncs with my other devices (iMac, iPhone), but any messages sent/received in between (i.e, between the test message and January 2018) simply don't appear on the old MacBook.

FWIW, on all devices I have conversations set to start from my phone number, not the email addresses associated with the account, though I have the same "you can be reached..." emails entered on all devices. I'm using the same iCloud account info on all devices.