I recently discovered that our payment processor does in fact accept a credit cared type that wasn't enabled before. I went to Administer > CiviContribute > Accepted Credit Cards and enabled the card (Discover). Then I cleared caches and CiviCRM caches but the credit card button isn't showing up. We are using Authorize.net.
In the screenshot below, you can see that it still only lists Visa, MasterCard, and American Express.
Update: Strangely, on my test site, it works just fine on 5.1.2 so when I upgrade production tonight, the issue might just go away.