I always sonicate at a 1:3 ratio (5 mins actual sonication time, 15mins off) per pellet from 200ml of culture. My samples are always kept on ice during the procedure too. The amplification was 40%.
I was trying two different buffers, one was something I have always used (tris, DTT, EDTA and NaCl at pH 8), the other was bought from Qiagen. But I didn't get a pellet for either once I had spun them down!
I've never had this problem before but I guess I will soon find out if this really matters soon enough!