My friend and I had the same issue. We spend an ungodly amount of time trying different things. We wanted to play the starter decks against each other. Here is what solved it in the end:
When selecting the decks my friend had the starter decks under "My Decks" as well as "Starter Decks". She had selected them from the "Starter Deck" section, when she instead selected them from "My Decks" instead, the matchmaking was working.
My friend and I had the same issue. We spend an ungodly amount of time trying different things. We wanted to play the starter decks against each other. Here is what solved it in the end:
When selecting the decks my friend had the starter decks under "My Decks" as well as "Starter Decks". She had selected them from the "Starter Deck" section, when she instead selected them from "My Decks" instead, the matchmaking was working.
Hope this helps someone save some time.