Iphone 3GS cant downgrade to lower Baseband

Hi Guys, I have an iphone 3GS, Running IOS 6.13, Baseband 06.15.00, the phone is factor locked, and I have to downgrade baseband in order to get the Ultrasnow Unlock on Cydia. I have already downloaded already downloaded Cydia on the phone, but the problem is when I have to down downgrade it, After selecting Downgrade Baseband, it says ''Baseband Downgrade failed'' Is there a way to fix this problem? I will appreciate your help guys.