Duolingo gives gold when you don't deserve it when strengthening skills.
I finished the French tree and I go back to practice using the strengthen skills timed practice and even if I only get 3 or 4 questions right it will give me 5/5 strength and move on to another section to practice. I realize that this may be on purpose so people can avoid frustration being stuck on one lesson but I feel like I am not retaining the knowledge from the lesson when it moves on so easily.
It is more likely because the gold or not gold of a skill is based on individual words and the since having been practised so if there were only 3 weak words which were holding it back and you got those 3 words correct then it wouldn't matter if you got the other words wrong.
If you feel like you do not remember much from the lesson then redo even if it is gold, don't let a color dictate what you do because you know what you know better than a computer does
I am surprised: Most people complain about the opposite thing.
Just strengthen it again. Sometimes I'll get none wrong at have to do it twice. Just strengthen it until you feel you have retained the knowledge. Don't trust the computer (well, on this particular case) It's just a square lump of metal, plastic, wires, etc. A pretty cool lump though.
Yes it can be fustrating when you strengthen something multiple times but this is also to do with the individual word strength, No matter how many you get right or wrong, if it is not the 'weak' word/s that you are getting then it will not turn gold, may people referring to this as the skill 'sticking' and can be resolved by going through every lesson in that skill to ensure you get all words.