MILDEW

There is mildew (mold?) around two air vents in the ceiling of our kitchen and living room. The vents are about six feet apart. The other vents are not affected.

There is fiberglass insulation in the attic. We have cleaned the vent registers and the surrounding ceiling area with bleach. However, the mildew keeps coming back in a matter of a few months. HELP! THANK YOU.