Message app

iMessages synch across to my iphone and vice versa from my iPad

But I have a couple of friends that don't have phone or apps that allow them to use iMessages

I have to use the basic apple message app (green icon).

These messages don't seem to synch across devices So if I send a message from iPhone using message app and it doesn’t change to the blue iMessage this message doesn’t automatically appear in my iPad under messages like the iMessages do.

Am I missing something on the settings