Italian lessons not use all new words
I was doing Subjuctive Imperfect Lessons and while doing number 5 which introduces "passasse, entrasse, lavorasse, vivesse, ricordasse, usasse, chiamasse" I never once got any sentence using passasse or lavorasse, despite getting the same sentence multiple times.
Also on Lesson 6, introducing "morisse, piacesse, guardasse, aprissi, seguisse, aspettassimo," I only got morisse, aprissi and seguisse and nothing else. I got the sentence, "She didn't want him to die" something like 4 times and right every time.
Duo selects words/sentences randomly from the words available in the lesson. That means that there is a chance that some words will get missed and others will get repeated. They could add in extra routines to overcome this problem but it adds to the overhead and Duolingo probably have other more important things to worry about.