I find that DuoLingo teaches many words once and then we never see such words in practice activities or subsequent lessons. The only way to find these "lost words" is to go back in previous lessons and review them individually or go to your word list and fish through all the randomness. Shouldn't Duolingo continue to flash such words in sentences of subsequent lessons? Retaining vocabulary is all about repetition. Spanish words like "arroz" and "libro" come up far too often while Spanish words like "red" and "calidad" are taught in one sentence and then forgotten about.
It could be because we don't use often those words. it's just my guess, usually we use calidad or red on specifically occasions when the topic is related with that. it will more useful if you write a list of words....I think that the best way to learn those words is using in a quotidian topic or make some sentence with those... Regards