Why Won't My QR Code Scan?
The most common causes, in order of likelihood: printed too small for the scanning distance, low contrast between the code and background, the quiet-zone margin was cropped away, or physical damage/obstruction exceeds the code's error-correction tolerance.
Work through them in that order. First, check size — a code that scans fine held close to the camera might fail at the actual distance it'll be viewed from once installed or printed. Second, check contrast — light-colored dots on a light background, or a busy photo behind the code, both confuse scanners even with the pattern technically intact. Third, check the margin — a design layout that crops right up to the code's edge removes the quiet zone the scanner needs to locate it. Finally, if a logo or heavy styling has been added, confirm the error correction level was bumped up to compensate (this site's tool does this automatically when a logo is added).