I had a similar issue using Mnemosyne. Then I read "Moonwalking with Einstein" and I'm pretty convinced that spaced repetition is a solution to an adjacent problem. More important is the vividness of the memory, and the use of spatial memory (memory palace technique), along with some other techniques, for engaging many parts of the brain. These make the memory stick much more clearly and for a longer time.

It's a great book but I understand there are others that are more focused specifically on techniques, with much less story.