The answer to 'why is my underlayment missing shingles' is that the underlayment is likely being damaged or worn away, causing the shingles above it to come loose. This can happen due to a few common causes, such as improper installation, age and weathering, or damage from foot traffic or debris.
If the underlayment is missing in patches, it may be a sign that the roofing system is reaching the end of its lifespan and needs to be repaired or replaced. In some cases, the underlayment may have been damaged during a previous roofing project, or it could be a sign of poor-quality materials or installation. Addressing the issue promptly is important to prevent further damage and leaks.