'You can now translate' message
I've reported this twice to Duolingo, but the problem is still not fixed. As I complete lessons in the Italian section, the message which appears at the end of the lesson contains a lesser, not a greater, percentage. For example, yesterday the message advised 'You can now translate 73.3% of all Italian text. Want to try?' Today, after I completed two additional lessons, the percentage went down to 73.1%. This must be happening to everyone who completes lessons in the Italian section, no?
I think it's because some of your words will have weakened and what you gained by completing the lesson won't have completely made up the difference.
I think you're onto something. Some of the lessons I completed earlier are now showing up as needing strengthening, but I don't think I've 'forgotten' words in those lessons and Duolingo has somehow figured that out! I suspect that Duolingo has made changes to those lessons, and uses an algorithm which automatically adjusts for the fact that I haven't done the new sentences in those lessons, when then decreases the percentage of text Duolingo alleges I can translate.
I don't think I've 'forgotten' words in those lessons
Words do just weaken on their own. How fast they weaken depends on how often you get them wrong. To my knowledge they never stop weakening though.