i reworked/reflowed many cell phones and notebooks - i always use some FLUX for those jobs - get a "Flux Pen" and use it before you start reflowing the board.
but be careful - the tip of the pen needs to be pressed on the board
don't pess directly on the chips!
use not to much flux - only enough that the chip seems to "swim" on the board.
another thing about the temperature:
you can see the perfect temperature on the surrounding parts - the solder get's "shiny"
a "dry" rework can sometimes lead to problems - so, get some flux and try it again -maybe you should read the wikipedia article about flux (the link is above) - it helps to understand what it does
i use "no clean - flux pen's" - easy to get, unproblematic to store and not that expensive.