suggesting 'quadrivial' for things hard to understand."

Anyone else had this happen to them ?
When an intricate, even arcane thing is explained to me I can follow and understand it , yea, even write an easily understandable exposition ... HOWEVER ... a day, week, month later I cannot explain it to anyone. I know what "it" means or causes but at a loss to explain to someone else as it was explained to me.
Is there a word for this? (she asked, neatly tying it to words!)
wow