Feynman technique fails when existing explanations are bad
The Feynman technique is great when you really want to internalize something. But the technique has a problem when all existing explanations suck. In that case you can try to explain it yourself, but when you get stuck, there is nowhere to turn to.
Even explanations of the technique use these bad explanations e.g. this site that explains the feynman technique using the pythagorean theorem as the example sucks: https://collegeinfogeek.com/feynman-technique/ -- this is hardly an explanation of the theorem, and they only show a proof at the very end and it's a rearrangement proof that doesn't give any insight.