Is it just me or have lessons been degrading far too quickly recently?
I always had no trouble keeping everything gold, but yesterday I open the app and like half of my lessons are no longer gold. I redo them, and the next day, the same thing happens. At this point 7 lessons fade per day. I need that trophy, yall.
Take a look at this, which might help you: http://gregreflects.blogspot.com/2014/12/how-to-make-gold-duolingo-tree.html
I've had a noticeable uptick in skills degrading in the reverse Spanish -> English tree. I've wondered if there might have been a rather subtle tree upgrade with new words added, but what I'm experiencing now doesn't really fit that pattern. It might, however, just be coincidence.
As Woof said, you do not have to strengthen anything if you don't want to or don't think you need to.
I've chosen to strengthen (at the rate of two skills a day) rather than advance further for now because a) at some point I decided I'm in this for the long-haul and doing the tree faster would just mean more practice after finishing the tree, b) I clearly need the practice for some of the skills, c) I know at some point the rate of degilding for skills I've already completed will decrease in accordance with the spaced repetition algorithm , and d) why not?
Other people are perfectly free to try different approaches.
The three likeliest explanations that I know of:
1) You're on the unlucky side of an A/B test. If this is the case, I don't think there's a lot you can't do to 'fix' it. There are ways to find out, but I don't have the tech knowhow. It won't change a lot if you do find out - you'll just have to be patient and hope that Duolingo don't think your side of the test is better than the other! - but at least you will know.
2) You're using hints or making errors when you strengthen. (This may seem like a duurrrr point, but some people remain unaware that both making mistakes and using the hover hints will cause Duolingo to mark a given word as needing review soon.) As you move down the tree, this becomes more likely (unless you know the material really, really well) simply because there's more to remember. Keep an eye out and be honest with yourself. It may be that you don't know the material that well yet and so Duolingo is prompting you to revise things it thinks you need to do more work on.
3) You have a bug of some kind. Redo all the lessons within a skill and then do a strengthen; for preference, do all these things without using hover hints or making mistakes. If your skill stays solidly gold. then it's likely (in my experience) that the skills are degilding elsewhere because you're using hints or making mistakes (or, possibly, you're in an A/B test and you'll have to grin and bear it). If your skill still won't stay gold, then report it to Duolingo as a bug.
Regardless of what is the root cause, also always be aware that there's no obligation to get your skills gold before you move forwards. (This is particularly important if you're part of an A/B test and it's stymieing your progress.) It's nice to have gilded skills, and generally speaking, it does tend to mean you know the material well and so your progress forwards will be smoother, but if your situation is 1 or 3 and it's making your progress grind to a halt, then if a skill loses its gilding and you know you know the material in that skill? Ignore it, do a new lesson.
You have zero obligation to keep your tree gold, and while the DL algorithm is helpful, it's not the be all and end all and it does behave oddly at times.
General rule of thumb for speed down a tree - assuming you're not being deliberately faster or slower, and just want to go at a pace that suits you: if new lessons are super easy and you make no mistakes, and/or you actually know some of the material that's supposed to be new, you can probably speed up; if new lessons take you a long time, if you get a lot of answers wrong, if it's not just the new material that's tripping you up, you probably need some revision; if you get through the lessons with errors/having to look at hints only in the new material, then your speed is probably about right. If Duolingo disagrees with you about the speed but you know you're progressing comfortably, ignore the lack of gold.
It's called user-retention on DL's part, it helps make their subscriber-base look better to potential investors ;)