One thing that often gets overlooked is the position and angle of our picking hand when playing something that moves across several strings.
I’ve met many students who were able to get a good picking motion going; but, for some reason, it would fall apart when applied to more than one string. When I took a closer look, I could see that they planted their hand in one place and kept it there, regardless of which string they were playing.
If you have a guitar handy, try it now. Plant your hand near the lowest string; when you move your pick from the lowest string toward the highest, your wrist deviates downward in order to let this happen. The side effect is that the angle of the pick is now totally different from its angle when you were on the low strings.
If you keep your hand locked in place, a mild, almost parallel pick angle on the lowest string will turn to an angle that is almost completely on its side on the high E string. So the way the pick connects has changed. Not only that: the angle of your hand also has changed.
On the lower strings, you might've been able to get a smooth, efficient picking action happening. But with your wrist pointed down and no longer in line with your arm, you’re having to resort to a different picking action, which resembles more of a swiping, "back to front" motion. So what happens is you’re expending energy moving the pick "back to front" while the pick edge struggles to gain purchase on the string. You’re clipping the string, producing an audibly different sound as well—a scraping tone that you might or might not be happy with.
The solution is simple. Identify