I quite like the expression "... eludes me" (or in French "... m'échappe"). I can express this with something like 分からない, 理解できない but this is not faithful to the expression "... eludes me".
Indeed, "... eludes me" describes something that you can't catch, the thing itself try to not come close to you so as not to be caught. That is the thing eludes you on its own. On the other hand, 理解できない or わからない only tells that you can't understand but misses the nuance that the thing tries not to be caught.
Is there a word in Japanese that matches this nuance?