It's strange, because what you want is already the default (all players' cards are used to choose from).
1) Are you sure you are using the account that you have subscribed on, and that the subscription hasn't run out (people get caught on this regularly)? You can check in the account tab, make sure that the ending date (given as year-month-day) is in the future.
2) Have you set your card pool level to 10 (as below), and the minumum to something higher than 1 (preferably also 10).
If that all checks out, can you post a screenshot of your automatch screen?