And “Fail” is not? 
“Again” has an affirmative-tone, focused on learning and trying again. (It also leaves room for “I want to see that one again” without regard to whether the answer was right or wrong.) I don’t think renaming the button to someting more negative is a good idea. And regardless of what name you put on the button – there will still be people who don’t understand what it means or how to use it.
If there’s something that will directly help the minority of users who don’t understand what the buttons mean, focus on that instead of changing the foundational parts of the app that work for most people.
[reluctantly hijacking your suggestion with a completely different proposal]
I remain baffled as to how people start using an app like this without trying to learn anything about it first. Help > Guide should be plenty – but it keeps not being enough to engage with some users. So, what can we do to get those users connected with the “Getting Started” page sooner?
What about a persistent welcome message/link to “Getting Started” displayed on the Decks page (or popping up on start) until a certain threshhold is met? We would need to find a point that will ensure it will go away when it is no longer useful and will not pester experienced users. What are the hallmarks of a “beginnner” to the app? [I don’t know what we already can “count” and what we can’t, so these might not be feasible…]
- Number of decks besides the Default deck?
- Non-empty Default deck?
- Total number of notes in the collection?
Amount of time with the app running or number of app “starts”?[probably better that it be focused on the collection, so we don’t have to worry about it triggering on upgrades/reinstalls]
EDIT:
I thought of other measurement criteria –
- Total number of reviews completed? [might be hard to calibrate what the right number is]
- Number of days with studying history? [thinking that if we haven’t gotten someone to read the page in a week, it’s probably target-gone-by]