Why does my iMessage on Mac no longer show contact names?

So a bit of a back story.

I had to change my email address for my Apple ID. Which of course signed me out of iMessage on my Mac.

I signed back in to iMessage and some of the previous conversations only have a number, not a name.

Sometimes I will see a names when I overlook all the conversation history, but when I get a notifications they come up with the number and a picture of the contact that I have created, but no name.

So for example, in Australia the area code is +61 If I have someones contact under 0400000000 saved like this, in the conversation history/notications it would show up as +61 400000000 and sometimes also show the contacts picture.

Normally it would convert the +61 even if I didn't have to put +61 in the persons contact. I've edited some contacts to add the number +61 but I shouldn't have to do this.

Now a few days later all my conversations are showing numbers only, and no names and no contact images.

Any help? 😟 on 10.13