I'd do a lit search to see why ppt might be happening.  Go through each step in the protocol and check that your buffers and all etc are correct.  Without more detail on what you are doing I cant really give an answer.  My guesses are  pH , temperature, contamination, electrolytes are off...hard to say--what would make it ppt'ate? 

Hello thank you for your reply. I prevented the precipitation. What I want to do now is separate monomer from oligomer in Size Exclusion chromatography. But I am not getting two separate peaks for monomer and oligomer. I am getting a sharp peak of high molecular weight corresponding to oligomeric species.