Well, try to lower your wrist, and use a small wrist motion, but also a finger motion. And try to get a really really tiny accent at each 2 bass notes. Just to get a relaxation point there. I used to use a TOTAL wrist movement, and no relaxation, so getting the C tremolo was impossible, but when I changed note at each beat, it was okay. This is because I get relaxation when changing note. Well try to do the same as if you would change note, but keep the same. That's how it worked for me anyway.