Check for cracked housings on plastic roof vents and broken seams on metal ones. You might be tempted to throw caulk at the problem, but that solution won't last long. There's really no fix other than replacing the damaged vents. Also look for pulled or missing nails at the base's bottom edge. Replace them with rubber-washered screws. In most cases, you can remove nails under the shingles on both sides of the vent to pull it free. There will be nails across the top of the vent too. Usually you can also work those loose without removing shingles. Screw the bottom in place with rubber-washered screws. Squeeze out a bead of caulk beneath the shingles on both sides of the vent to hold the shingles down and to add a water barrier. That's much easier than renailing the shingles.