Strengthen the same category five times in a row?

[RMB229]

This has happened to me twice: The strength bar for a certain word type is at 4/5, so I want to strenghten it to 5/5. But I have to re-do the lesson several times before I get there. After successfully finishing the lesson - sometimes without any error - I do not gain an extra strength point. It seems random, since for other categories it works fine.