Greek - future perfect
This one will not strengthen - no matter how many times I do it. This is not very motivating. I understand that the course is in development, but is this not something that can be tweaked?
Not by the course maintainers - there are a couple courses that have such a "resistent" module, so it seems to be some kind of underlying Duolingo problem.
What has helped in the other courses is not strengthening the entire skill, but redoing the single lesson inside the skill.
(In the other cases I've see, the buggy skills have all been single-lesson skills. I'm not sure whether that's the case for "future perfect" as well. If not, you may have to manually redo each individual lesson separately in order to regild the skill.)
Phil, I've got the same problem - none of the three future modules (Phrasal Future, Future Simple, Future Perfect) will strengthen for me at all, no matter how many times I try. Occasionally, they'll strengthen when I'm doing a completely unrelated module, however.
Mizinamo - many thanks for the tip, I'll give that a go.