I'm not sure if this is possible at the moment. If not, I'd like to see it be a feature. For example, if it takes you >10 seconds to answer a flashcard, it will repeat it after 'x' time. I feel this would reinforce those answers that you had a harder time answering. I know you can answer "barely remembered" but I usually don't feel too confident if I'm only questioned about one card once a day. I know this is problematic if you are trying to review a lot more vocabulary but to have the option would be nice.