If you find yourself getting stuck on the same words over and over so that you have to relearn them 20 times than those are Leech Words. They leech your time, energy and enthusiasm.
As far as I can see Memrise’ algorithm does not deal with such word, so I just “ignore” them and make a note to review them. For these words, I dedicate specific time and method, trying to build emotional context, mnemonics etc. To really understand their meaning, how they are built etc.
As a side note, I also set retention rate at 90% for my own local app, as memrise does not have this option it’s better not to push too fast with new words if you see review words accumulating. Better to slow down, go over those leech words, use other methods such as reading, youtube, let it all sink in, and then continue.
I would also suggest using that script that makes you type in the answer in the target language. Also, strictly avoid Speed review. It gives you the illusion you know the words but in fact they disrupt the whole Spaced Repetition idea. To learn words you need to recall them and if you see them you obviously did not use effort to recall. It is counterproductive.
This works for me, hope it helps a bit.