I have been completing my daily classes on the French course for months now. I spend about half an hour every day. I understand the need to revise lessons often but almost all of the lessons right back to basics loose their golden colour every day. This is most disheartening. I feel I can't make any progress.. I also paid a while ago to repair my streak. This had no effect so I don't know why you ask people to do it. Thinking of giving up.
You do not necessarily have to strengthen something just because it is not gold, if you feel that you know that skill very well then leave it and move onto something you feel you do not know well, even if it's gold. The algorithm is based off of averages of how much time passes before you forget a word, Duolingo follows this average so makes the skills non gold based on averages of when people will have forgotten. However with averages there is always a lot of people who do not fit into that and so this method will not work for them, it is fairly likely that you are one of these people so if it helps you to not follow Duolingos algorithm then you should do that and focus on the ones you know you need to improve on.
Have a look at this: http://gregreflects.blogspot.de/2014/12/how-to-make-gold-duolingo-tree.html
Thanks all, I'll keep at it but still can't understand, if I strengthen a lesson and get it all correct and turn it gold, why does it loose strength as soon as I open the app again. It's often just later the same day.
Make sure that when you're answering questions, you're doing it without clicking on any of the words to check what they mean. I found that really affected how long my skills would stay gold.
With enough time and practice you can get to the point where they do not weaken so quickly. I rarely have to strengthen anything in my French tree anymore - I would say on average 1 module every day or two.
Don't give up! You can do it :)