getting stuck on "entering recovery mode" in blackra1n

I'm trying to jailbreak and unlock my iphone 3g with firmware 3.1.2 with blackra1n. Every time I try to make it rain, it goes into entering recovery mode" and stays there. The iphone itself turns off and then shows the usb to itunes picture. I can not get this to work for the life of me. I've tried so many solutions and looked everywhere for what to do and nothing has worked. Please help! Thanks!