it should still be working on you, the jitters seem to be from the onset of it mainly. You could always experiment with it and up the dose if you feel the need, I have heard of some people on this board taking crazy doses in excess of 800mcg/day, I wouldn't personally take much more then 240mcg/day.
Ive taken clen to the max at 120mcg and had good some results, im not a big fan of it though, i think ephedra is far superior for fatburning, and you dont have too cycle on and off. For my competition diet right now i havent used clen at all.