I practiced a Spanish skill (Adj. 1) twice. One time I missed one question, the other I went perfect. The first time it improved one bar. The second, not at all.
Is this a glitch, a bug, or just my incompetence?
Let's imagine you started with just one bar, but that one bar was really full, almost to the point of two bars. So the first practice you did a few words and that tipped you over the edge into the second bar. So you got visual confirmation of your progress.
Now you see two bars, but you are just barely into that second bar. You practice again, getting everything right, and it fills that second bar up some, but it's not quite enough to tip you into the third bar.