A common place where leaks occur are around roofing vents. These are designed to let air flow in or out of the building, but not to let rainwater in. So if water is managing to find its way in around these parts of the roof, it should be fixed as soon as possible.
To fix the problem be sure to 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 out for pulled or missing nails at the base’s bottom edge. If this is the case, 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 which you can work those loose without removing shingles.