I’m not sure if this is helping, but maybe it does. I sure hope it does.
In my experience, everything depends on what and how you are learning. And by that I mostly mean the structure of the course you are working on.
When I’m using just any course, thematic lists, sometimes I have trouble remembering things and sometimes it goes all right, it depends mostly on how tired I am, how strange the words are, and a lot of other factors which others have already mentioned. But the method which works best for me is a bit different. See, I don’t really like mems. They just don’t establish a strong connection for me. Instead I need some other kind of device to connect words and meanings.
That’s why Memrise is not my primary/only source of learning but a very important complementary part of the process. I’m learning the language, so I’m reading texts (either in a language book or a short story, a novel, an article, anything we are working with in school, anything I found online or in the library or really, anything I’m interested in.) First I only read it to have a basic idea what the text/story is about. Then I read again with a pencil in hand, underlining the words and phrases which I don’t know, or which feel strange for some reason or which use a word in a new or unfamiliar meaning. Then I build a new level of those underlined phrases on my own course, definitely not using a wiki, but rather figuring out and researching their exact meanings, looking it up in several dictionaries (monolingual and bilingual), maybe checking out how the word is used in sentences, finding synonyms if necessary. Just getting familiar with it. Then I can add a well thought-out entry to my course database. (And while I’m doing that, I can also check out the existing similar entries, which reminds me of those ones as well.) Some people swear by adding audio too, I for one find that annoying, it just distracts me from learning, but that might be a personal preference.
So when I’m learning the word/phrase, usually a few days after creating the level, it is already familiar, because I’ve read it a few times, I’ve spent some time with it, I remember the context it was used in originally. Then I go through the needed six repetitions when planting, and the first watering usually the same evening, and the second the next morning. By that time I feel like the word is, if not exactly my friend, still an acquaintance I’ve met numerous times. I can definitely recall knowing it from somewhere.
(Another plus with the own course is, if I find a mistake, if I learn something new about a word, if I find that my clue is misleading, I can always correct it. Another great thing is, that I have a log of all the words I’ve encountered. This also gives an impression about their stylistic value based on which types of texts they occur. I still use other people’s courses though, their main value being that it’s another language pairing which often showcases a new nuance in a given word, and also that it was someone else doing the thinking which I can contrast with my findings.)
I know from experience that if I want to score above 90% on a word test in school I need at least four days, but preferably a week to spend with those words. There are a few in my class who manage the same feat by cramming only the night before, but I don’t know how much they remember a week later.
It still happens of course that I forget something (usually after the longer resting periods, when I haven’t seen the word in a while), but then when I see the right answer, I immediately recall not only the word itself, but also the text it is coming from and a bunch of associations, maybe even the emotions I had when reading the story. So while this is a very slow process, much slower than using someone else’s course, but I feel it is worth the time and the effort, and it pays of in efficiency.
That’s something I keep recommending to people (also in school, I tell them not to use my course and build their own instead, not that they listen) especially if you have trouble remembering after such sporadic exposure. So building your own course based on your own learning and own experiences might be something to look into. And if you feel too self-conscious for it, you can have your course private or unlisted, only for your, it doesn’t have to be public.
So whatever you decide, good luck with your studies - and keep in mind that training your memory every day makes it actually stronger, so even just 15-20 minutes memrise-ing a day increases your capacity for new info radically.