Depending upon how much air is leaking, you might have a situation where the lid or bowl are out of round or out of 'flat'. Try turning the lid around on the bowl to see if it seats down better in one position rather than another.
There are some grill engineers on this board who could give you lots of technical advice about how to assess/correct the problem, but if the grill is new, I'd suggest you call Weber Customer Service to see how they can help.
But in any case, sti is right about the bottom like: if it's making good food, you are good. ;-) Welcome to the WKC!