The answer to 'why is my flashing missing shingles' is that the flashing around your roof's edges or penetrations has likely become damaged or detached, causing the shingles to come loose. This is a common roofing issue that can be caused by weathering, poor installation, or age. Symptoms include missing, curled, or loose shingles, as well as potential leaks or water damage.
The solution may involve re-securing or replacing the flashing, and potentially replacing any affected shingles. This is a task that can sometimes be done DIY, but it's often best to call in a professional roofing contractor, especially if the damage is extensive or the roof is older. They can properly inspect, diagnose, and repair the issue to prevent further damage and leaks.