'Practice weak skills' does not update some skills
I recently set out to bring my French tree back to gold after ten weeks of neglect, using 'practice weak skills', mostly in the iOS app.
This works fine except that four skills just will not show any strengthening at all, and are still at 1 or 2 bars when all others are gold or 1 bar off gold. And I have answered questions with the relevant words correctly many times. The four skills are
Être/avoir (though bizarrely the words listed as weak are only " ce " and " c' ", not a verb in sight)
Very similar things seem to happen in other languages.
I know of course that I could just practice these skills directly, but my question is WHY the 'Practice Weak Skills' algorithm is not working for them?