First off, what are you using to meter? If all you're using are the track meters for Pro Tools, you may want to add in some other metering method for verification (even if it's just a plug-in on the Master Bus). In general, I don't trust DAW meters. I like to have at least one more for verification.
Second, I have to echo Utopia's question. Why do you want things to hit zero? I've never met a mixer who wants to peg out their audio signal. Almost everyone I know leaves at least 1 dB at the top end of their mix (and those are in the most EXTREME cases).
The most important question is: "Does it sound good?" When you're talking about the overall mix, no one is going to hear .5 dB or less of difference if that's what you need to get it up to zero. If it sounds good, and you're making good use of the dynamic range, just leave it as is.