has anyone else had trouble with the wheels breaking off of their stools? i've owned two different stools and both of them have had the wheels break off. it seems as if you should be able to just screw them back on but it doesn't work. i put them back on and 1 client later, it busts back off. what's the deal? the first one it happened to i only had the chair for about 4 months. the other, about 1 year. both stools cost over $100 so they weren't inexpensive ones! what's the deal?