The answer to 'my flashing is failing' is that the flashing around your roof's edges, vents, and penetrations has likely deteriorated over time and is no longer properly sealing and waterproofing those areas. Common causes include age, weather exposure, poor installation, and damage from roof repairs or modifications. Symptoms include leaks, water stains, and visible cracks or gaps in the flashing.
The solution is to replace the failed flashing. This is generally a job for a professional roofer, as proper flashing installation requires specialized tools, materials, and roofing expertise. Attempting a DIY flashing repair without the right skills and experience can lead to further leaks and damage. A qualified roofer will be able to properly remove and replace the flashing to restore a watertight seal around your roof's edges and penetrations.