I personally think you probably did something wrong, but I can’t say exactly what beyond ‘not making the connection between meaning and kanji as immediate, obvious and intuitive as possible’. That’s what I aim for. I’m not going to claim I’m a memory expert, but I remember things fairly well provided I want to (I use a lot of vivid imagery), and I’m a native English and Chinese speaker, so I’ve been doing this with kanji for a long time.
Could you perhaps provide an example of a Heisig mnemonic that you still have in your head, along with the kanji for which you’re using it? Feel free to pick a complicated one/one that you struggle to recall quickly. I’ll give you my opinion on what’s wrong or how you could approach it differently.
I’d say that this is also a possibility, if only because having a reading might provide you with a more concise trigger for meaning than an entire mnemonic, and you can use readings to strengthen your memory of meanings. However, as long as you have no meaning keyword overlaps in Heisig, you’d essentially be ‘reading’ each kanji as its meaning, so I doubt that’s the main issue here.
If you want to take a look, I have a thread where I create mnemonics, primarily for other people, but also for a few new words with complex kanji that other people have made me aware of. It’s here:
I aim to use emotions and vivid sensory imagery as much as possible (though I’m aware that not everyone can visualise or imagine things) in order to reduce thinking time and make meanings and readings intuitive. The only reason my mnemonics are so long is that I’m trying to describe exactly what I see and feel in my head so others can (hopefully) experience them too.
PS: I don’t mean to make you feel uncomfortable or like I’m singling you out, but is French your native language, or is English also one of your native languages? (It’s nothing to do with your English, which is perfect: it’s just that I noticed ‘secondes’, which was surely due to autocorrect or a typo, your name and your punctuation for the colon after ‘PS’. I speak French and I’m currently living in France, so I couldn’t help but see all that.) The reason I’m asking is… well, could it be that you’re working with something that’s not in your native language? That of course depends on how comfortable you are with English, and it’s probably not the problem if you’re very used to English, which seems to be the case, but it’s just a thought. I speak French fluently, but I’m probably just a tiny bit slower in French than in English, and it probably wouldn’t help if I was trying to remember something convoluted in French (like when I take notes in French literature class and the teacher doesn’t repeat herself ).