I'm currently having issues with my Sram Force rear derailleur. I'm running a 53/39 crank along with a 11-27 cassette. When I'm in the 39 up front and start to work my way down the cassette towards the 11 in back the chain falls off the little wheel on the derailleur that's supposed to guide it. When I back pedal it comes back on. Does anyone know what the problem might be or have a solution?