Watch the Trickle online guitar lesson by Anthony Stauffer from 50 Slow Blues Licks You MUST Know
Lick #37 is very simple. You won't have any trouble playing it once or twice. However Stevie Ray Vaughan didn't play it only once, he repeated it many times in a row. There's something about the order of these pull-offs that makes it really tricky to play repeatedly. Try playing it slowly and deliberately until you get the basic motion committed to muscle memory, then work on playing it faster.