If you bought a motion-activated switch for fluorescent lights in your garage or other locations and the lights didn't work, your switch is probably rated for incandescent lights and fluorescents with magnetic ballasts. Unfortunately, it won't work with fluorescents that have electronic ballasts, which are now common in better-quality fluorescent lights. (A ballast is the small metal box that modulates voltage in a fluorescent fixture.) It's irritating, but you have to read the fine print on the packaging to discover this limitation.
To find a switch that'll handle both types of ballasts, you'll probably have to go to an electrical supply store (search online or in the yellow pages). Our example, the Leviton model No. ODS15 (or ODS10), costs about $60. Most low-priced motion-activated switches ($15) that you find at hardware stores and home centers aren't rated for electronic ballasts.