Skills don't decay.
At his moment I am around last checkpoint and my skill are not decay at all. I didn't see any decayed lessons for a month.
How do you solve this problem ? What system of repetitions do you use ? Are you writing some scripts to keep decay system alive , then what formula for decay do you use ?
P.S. There was no such problem when I was finishing my French tree.
P.P.S. Does it help to reset a course ?
Yes and no. Initially, I felt my skills were indeed decaying too fast, and that I was forever doing the same "Maria is from China; John is American" stuff. But for quite some time now, I have had the same problem as LightMor: skills just don't decay, and this does in no way reflect my actual skill level.
So I have created a file on my tablet to let me know where in the review process I am - otherwise I lose track.