The teaching method
Lost interest? I have done my exercises and got the colour around. When I paused for a day I discovered that one colour part had gone from some completed exercises. I do not want to repeat them again. It puts me off. Why is it so. It is not very encouraging.
To learn a language you have to practice a lot, since you forget things over time! Duolingo takes this into account, so after a while your gold skills will change color to show it's time to practice. You're free to ignore it, but if you really want to learn the language it's a good idea to repeat!