'Strengthen skills' is not raising any skills up to gold?
I went away from Duolingo for several months and recently picked it back up. Obviously, my skills in the trees degraded over that time, so I wanted to restrengthen them. After getting about half of them back up to gold, though, using the strengthen skills button isn't raising any skill bars anymore. What's going on here?
In spite of its name, the Strengthen Skills exercise does not target skills for improvement. It targets the weakest words in your vocabulary. If you sort your words list by strength, you'll more than likely see that the weakest words are spread across many different skills. The strength of a skill is merely the average strength of the words taught within that skill's lessons. When you do a practice exercise, 9 words are targeted for improvement. Even if by some stroke of luck all nine of these happened to belong to the same skill, most skills would require more than one or two practices to raise the average strength of a skill's words high enough to show full strength (80%+ average word strength).
The fastest way to raise the strength of a skill is to, as Usagiboy7 mentioned, do the skill-specific strengthening exercises on the various skill overview pages. Even then, though, be prepared to do multiple practices on highly degraded skills, particularly in the jumbo 10 lesson skills that have 70+ words and to strengthen (even more word forms - plurals, gender-specific variants, various conjugations, etc.). For example, if you've got a skill with 70 words and they're all fully degraded (all words at near zero strength), you'll have to do 7 skill-specific practice sessions to get it back to gold. It takes a lot of work to get a neglected tree back to fully golden.
See this recent discussion in which I describe this in greater detail if you'd like to know more.
If you've been gone a long time, the skills will have degraded a lot. So, you will need to refresh them multiple times to get them to strengthen. Don't give up! PS, also, try entering a particular skill that is not gold and doing Strengthen Skills from inside of that skill. You might need to redo it 5 or so times.