Spaced repetition algorithms are a minefield. I have developed and partially validated an unpublished (so far) SR algo. The biggest failure of existing SR algorithms and something which has been studied in literature since 2016 a phenomemon in the human brain where retrieval performance of an item you want to learn becomes easier and harder seemingly at random. It turns out that there are three distinct phases of memory and your brain switches between them over time as it consolidates things you learn. A regression of performance over time becomes complicated. There are ways to deal with this using regime switching markov decision processes, but it is incredibly easy to get it all wrong. Another essential component of spaced repetitions so many algorithms get wrong is ignoring the spacing effect.