All of a sudden half of my duolingo tree has lost it's strength over night. Has something gone wrong?
might be a coincidence OR the team is testing out a new decay algorithm on some of the users. I had a few decayed skills as well (3-4) and normally nothing ever decays for me.
It's always a good idea to list your operating system and browser each time you post to Troubleshooting. (For example, mine are Windows7 and Firefox). :)
I'm not staff, so I can't see as much as they do. But, from what's showing up in your general Activity stream that all users can view, it looks like you were most active 6-7 months ago and have only recently come back to do a lesson. If that is so, you will need to do a lot of practice to strengthen each skill. If you only do a little practice to make it turn gold, and if you miss questions or peak and words, all of this will likely lead to faster decay.
If you have been practicing your skills a lot without peeks or errors and the tree is still not turning gold, it could be a glitch. Either way, I hope it all works out ^_^