Skill strengthening defective
I just tried to strengthen "Household" in Spanish because it was 1 bar down from gold. I have done the "strengthen skill" option FIVE times, and it hasn't gone up at all. I think that there must be a bug here.
What should definitely help in such a case is redoing each of the six lessons. I would also guess that after doing that you are safe from the bug in the Household topic, at least for a while.
I usually just use the overall skill strengthening button. With that I have noticed what may be a similar effect for some lessons which just take much longer to complete than the others. But it's less obvious in that case as there may be 'urgent' little things in other lessons which still look complete overall and so are presented first.
I just had this effect in Countries. But strengthening the Countries topic itself just once then did it.
I actually tested out of the entire tree the first time through, and just wanted a quickie review by refreshing each of the lessons in order again. Definitely not worth the time to redo each section individually for something I already know. Just want to de-bug Duo if there is a bug involved. :)
I have just completed french strengthening adjectives exercise but the programme will not allow me to 'submit' at the end of the exercise or to note an 'other' problem. So the only option is to quit and lose the lot. This is the 2nd time this has occurred on Duolingo although I don't recall which exercise it happened with before