I don't think the progress algorithm is properly reflecting the level of effort put into practicing a language once the tree has been completed. When I first started using Duolingo French I plowed my way through the tree doing 5 units per day. By the time I had completed the tree I had achieved about 40%. I carried on practicing at a slower rate of about 2 or 3 lessons per day with the lessons all gold but the percentage wouldn't budge. Eventually I gave up for a few months and didn't practice at all. When I next looked at Duolingo most of the lessons had changed colour and were asking for more practice so I worked my way through the tree again and this time every few lessons I would improve my progress score. Having turned them all gold again my score is 68%. That was several weeks ago and now I'm doing 2 to 3 lessons every day and it doesn't make any difference if I chose a lesson or if I just hit Practice the score doesn't budge but I think i'm making less mistakes. It seems that once all the lessons are gold practicing doesn't affect the score very much. Oh and whilst I acknowledge that we are trying to learn a language not score points the progress score is a useful indicator that the time spent is having some effect particularly for those like me who have no interactive means of measuring progress.
There are lots of posts about how the fluency % is not meaningful. Last year I worked for weeks on my French tree and my fluency remained at 0%. The finally after almost finishing my tree it went to 1%. The next day it went back to 0%. It was a bit deflating.
Now the fluency % inflated- All I have to do is watch a movie in French or German to know that I'm not fluent. :)
Having said that I do like seeing the % increase because it serves as a motivation for me (even though I know it isn't accurate). I understand how you feel about the % not changing.
I also wish that the algorithm measured something different - but hopefully you can tell that you are becoming more fluent, as the lessons become easier and easier - and translations come naturally.