The answer to 'why is my gutter guards missing shingles' is that the gutter guards may be causing damage to the shingles on your roof. Gutter guards are designed to prevent debris from clogging your gutters, but they can sometimes put additional stress on the shingles, causing them to become loose or fall off over time.
This can happen if the gutter guards are not properly installed or if they are the wrong size for your gutters. It can also occur if the gutter guards are not properly maintained, with debris buildup or other issues causing them to put pressure on the shingles.