Spaced repetitions only work if you use them every day with minimal or no breaks. If the algorithm actually does the recall probability very well like FSRS does, you will keep failing the cards if you don't do them consistently. I learned the hard way where I almost forgot like 80% of my spanish deck that I was certain that I will be able to retire and recall it. But nope, even that word that you felt was rock solid in your memory is gonna fade, so just trust the algorithm.