first of all is your phone rooted? because if it is there are blocks built into the root so that it doesnt finish the update.
Another thing make sure that you dont have any frozen apps or that you have all your apps updated so that the update goes through.
What blocked me was my frozen apps. I couldn't unfreeze guided tours 2.3.4 and that blocked me from updating. Just did a factory reset with the 902 fxz and it was no biggie.