Have you pressure tested your noob and determined that you have a leak at the crystal?
Like Joe says, you can repress the crystal w/ a crystal press. Beyond that, you're on to replacing parts- maybe BK could get you spare a gasket and crystal- or you try to silicone or glue the stuff back in there.
But I've found my two noobs to be WR- grease seals and tighten caseback.
I figure if you get 5 atm's, you're doing well, these are generally fine for swimming or light snorkeling. If you're into serious diving, buy a $200 dive watch like a Seiko! Better yet, get a dive computer!