The old style magnetic ballasts can lose voltage in humid environments which can prevent the necessary arc between the ends of the bulb. Either replace with a fixture with an electronic ballast, or alternatively, cut a narrow (1/4”-1/2”) strip of foil and attach it to the back of the bulb from end to end but stopping just short of the metal on each end. This helps a low voltage ballast make the initial arc required to light the bulb.