If you don't see a subscription on your Subscriptions page and you have already added the bank or credit account that it is charged to, you can add it from your list of transactions.
To do this, go to your Spending page. Find the transaction that is also a subscription. Click on the gray icon to the right of the charge and then select This is a subscription from the drop-down menu.
Refresh your Subscriptions page and you should see your newly-added subscription on the list.