Unable to strengthen some words
There are some words that I cannot seem to practice. I have strengthened the skill many times and redone the lesson many times as well, but they never show up.
I am having the same problem. It is not a universal problem, only affecting certain words. For example, I need to strengthen the noun Essen, German for food. However, the exercise to strengthen it also includes the verb essen, to eat. I noticed similar with the 3 other words that refuse to improve (Kleid v Kleider, Insekten v Insekt, unsere v unser). I believe the Duolingo algorithms are looking at spelling not the specific word. It is an educated guess so I could be wrong. Essen did strengthen to full briefly however after a few hours it was back to 1 bar.
I should not need to do timed exercises or use the mobile app to improve my strength. I prefer to not use a mobile device but rather my computer.
Since you are German 9, you are probably having issues with the same words.
Edit: In the 14h since I originally posted, unsere is no longer showing as needing practice. Another word that showed up today, Namen, disappeared after one practice session. The others are showing multiple days since last practice despite practising each several times within the past 15h. Duolingo needs to look into this. I have a number of people complaining about it.