When I check out my vocabulary list sorted by the strength with the weakest words on the top, and then proceed to do a "practice weakest words" exercise, some of the words on the top don't show on the exercise but only the ones under them.
Currently I have "que", "bientôt" and "chose" on the top of the list (and at least "que" has been there for quite a while...)
I have a similar issue. There are words that I haven't seen in weeks, are marked overdue for practice, and yet are never chosen for practice, even when I specifically choose practice weakest words.
It seems that the algorithm that decides what to focus on is skipping over them for some reason.