I have an iPhone 3g running iOS 4.2.1 with no SIM card. I'm attempting to jailbreak it so I can use it, but it will not enter DFU mode, even though I have tried repeatedly. I am attempting to use redsn0w 0.9.15b3, but my mac should recognize an iPhone in DFU mode anyway, so the jailbreak's probably not the issue. The home button is connected to the board, I opened it up and checked.
Please do not just parrot the instructions from the redsn0w jailbreak, as I have already read them many times. I am looking for advice on what may be preventing the iPhone from entering DFU mode.