I have had this problem before with mine as well. Your grooves on the inside of the knob are wearing off and possibly on the spigot as well. You will need to turn off the water to your entire house at the main valve, pop the little metal center piece out with a knife or screwdriver, unscrew the screw inside and take the knob off and and see if the grooves are worn down. To get the water spigot out you will need a special wrench tool that you can buy at the hardware store. Just tell them what you need and they should find the tool and also tell you how to take it off and replace it. You will need to bring the spigot in and compare to get the proper fit. Hope this helps. Roze
Semper Fi (Always Faithful) Proud Mom of a United States Marine