My tree isn't golding ...
even though it says I've strengthened the lessons I've reviewed. I've cleared the cache on my browser and still nothing.
Which lesson? Is this on the web version or one of the mobile apps and which one?
Which browser are you using? I know that there are many lessons for each section and the strengthen may work on one or more of the lessons and others within the section still may need to be strengthened. Some have 10 lessons. So it can take awhile. When you get every question right - how many questions did you answer? On the timed version, there are 20 questions to answer within the time limit. On the regular version, 17 questions means that you got them all right, but 26 for example meant that you had some wrong. Try redoing the lessons instead of using the strengthen to see if you can get past this. If this is the same, please request help here: https://www.duolingo.com/help (Scroll all the way down. Trying what they say first and then there will be place to request help.)