The answer to 'why is my drip edge missing shingles' is that the drip edge may have been improperly installed or has come loose over time. This can happen if the drip edge was not properly secured to the roof deck or if the nails/fasteners have worked themselves loose. Missing or damaged drip edge can allow water to get behind the shingles, leading to leaks and other roofing issues.
To fix this problem, you'll need to inspect the drip edge and re-secure it if possible. This may involve re-nailing the drip edge or replacing any damaged sections. If the drip edge is in poor condition or the issue is widespread, it's best to call a professional roofer to fully inspect and repair the problem.