Why won't Duolingo let me do strengthen skills?
When I try to strength skills, duolingo comes up with a message saying "you have practiced all your words, why don't you do some lessons or translations" (I assume it means immersion).
However, when I go to the words list, there are about 630 words which have only one bar of strength, and it says that they are overdue for practice.
How can I strengthen these words? Doing lessons does not help, as the words rarely come up in the questions. I don't want to do flashcards, as the wrong words come up, and then they decay back to 1 bar after just 24 hours. (For background information, I have completed my tree, and kept it gold).