Oyster needs light to form pinheads. The manual gives 200 lux for 8-12 hours a day; the folder asks for 6-8 hours from glazed windows or tube lights. A lux is a unit of light on a surface; 200 lux is a bright room, far below sunlight, and direct sun heats and dries bags.
Too little light looks like too much CO2: long stalk, small cap, poor yield. Below 100 lux the caps go pale yellowish; in very bright light they turn dark brown or grey. When you see long stalks, add air time first, then check light hours before blaming the spawn.