Yes, it happens. Sometimes, I can tell why a mnemonic didn’t work. But sometimes, logical, clever mnemonics I like won’t stick, and others (which are « badly » constructed) work and I don’t understand why.
Honestly if you are using a large volume of mnemonics, it’s better to not lose time perfecting your mnemonics, better 10 spontaneous «quick and dirty» mnemonics, than 1 really well thought out. A certain number of failures is normal, I think. Just throw everything you can at the wall and see what sticks. Unless your retention rate is dramatically low, forgetting is perfectly normal (even if frustrating).
About repetition:
It happens quite often to me that I fail to remember a word or kanji several times, but then, after seeing/hearing it in another context, it clicks and I remember it. Don’t forget that giving the right answer to a vocabulary card only means you are good at answering that card. And that’s not actually that important. The repetition should happen not by seeing the card more often, but by seeing the word more often in other contexts. Then the card will be easier.
Maybe if you feel frustrated it’s time to do less cards (by all means do all the reviews but slow down new cards, even suspend the ones which are frustrating, that’s why the leech-function exists) and spend more time consuming content you like. More fun and less frustration leads to better learning.
I feel that for languages spaced repetition is a VERY useful tool, but it can also be a trap, if I get stuck in an endless loop of reviews, which leave me no time to actually use the language. Now, this is a personal preference perhaps, but I feel it is much better to have a more relaxed view of cards when learning vocabulary. If I forget a word several times, I just ignore the card. Either I will encounter the word in my reading/watching somewhere else and «review» it there. Or I will never encounter it again so I doesn’t really matter if I didn’t learn it, because I didn’t really need it.
Also don’t forget to vary your learning methods. I found that adapting my learning routine every few months gives better results. If kanjis frustrate you, concentrate on other aspects of the language for a time. I had a period where I hated kanjis and stopped actively learning it and concentrated on other things. Then last year I felt I was ready to have a go at it again, and sat down to think about what were the pain points, and made my own method to address them. Maybe some of the ideas / thoughts will help ? See My system for japanese kanji