It happened to me many times already, after i learn some lessons, only some words from them appear in the vocabulary, and I can't see the other ones.
The vocabulary seems to be a bit of a mess right now indeed, the sorters don't work very well either (specifically the one that should sort on how well you know the word)
Little edit: there are dozens of duplicate words in my vocabulary, what's going on DL? :)
The Directions skill appears mastered for me but there are only two vocabulary words. It has been like that for a while. Could you check it out?