I did one line for a test Friday and most of the kids did well. If I have a week that the majority don't I do the test again the next week. My general goal is that 80-90% of the class are scoring an 80 or better on the test.
Some years the kids move faster than others too. This years woodwind class for me is a lot more with it than last years. My percussion class not so much... Worry about successfully teaching the lines. If they have them down move on. Don't move on just because you think a few are bored though. Truthfully I think if you constantly keep them playing I rarely feel like 6th graders get bored.