Software: Firmware Upgarde Issue: Will not upgrade to 3.1.2 From 3.0

Hello my fellow ifixit community. I have a question which perplexes me. I have an original 8gb iPhone. I just updated the firmware from 2.0.2 to 3.0. Yes i know, it was long overdue, but this was for a client that needed their phone jailbroken and unlocked. After this was completed, i decided to update the phone firmware to 3.1.2. I tried over 10 times (13 to be exact. I tired with various versions of 3.1.2. (original firmware from apple, custom firmware, and firm downloaded from 3rd party sites), but the firmware didn't go through. Various different errors came up. I thought to myself, maybe i have to upgrade this firmware by firmware (Ex. 3.0, 3.0.1, 3.1, 3.1.2, etc.). This didn't work. The only firmware that would download into this phone is 3.0 . This phone has never been dropped or open. So i ask you, my fellow ifixit community, what could possibly be the problem.