Sometimes when I fire up Duolingo I find I have incomplete bars on lessons that were previously complete. I don't mind going back and re-doing lessons, but this time I had over 20 lessons that I would need to re-do to have the gold bars filled in. Is this common? I can use the practice, but 20 lessons?
Yes words decay after a while I must say though I have never had to redo 20 before, when one starts to decay I click on the practice known words button or whatever it is called
Maybe duolingo made some adjustments to the word strength algorithm. That sometimes has quite a large effect. Also they usually test new things on a smaller group of users before implementing it for all the users.
You can also click on "Practice Skill", you don't have to re-do each lesson individually. Depending on how much the skill has degraded, you might have to "practice skill" more than once, but it is more efficient than re-doing each lesson individually. That gets one lesson at a time back to gold, while "practice" usually bumps up two or three lessons at a time.