To fix it, you need to eliminate any light leaks or sources of light that can interfere with your plants' dark period. You can use blackout curtains, duct tape, cardboard, or any other material that can block out any light from entering your grow room. You should also check your timers, fans, and other equipment and make sure they are not malfunctioning or emitting light. You also need to maintain a consistent and stable light schedule for your plants during their flowering. You should follow a 12/12 schedule for most strains or a 10/14 schedule for some sativa strains that require a longer dark period. You should avoid changing the light schedule abruptly or frequently, and give your plants at least a week to adjust to any changes.