The answer to 'why is my downspouts missing shingles' is that the downspout is likely causing water runoff that is damaging the shingles around it. Downspouts that are improperly positioned or clogged can redirect water flow in a way that erodes the shingles, causing them to become loose or fall off.
This is a common issue that can be caused by a variety of factors, including heavy rainfall, clogged gutters, or a downspout that is not long enough to divert water far enough away from the home's foundation. In some cases, the downspout may also be positioned too close to the roof edge, causing water to splash back up and damage the shingles.