New Flat Rubber Belts will all be shorter in length than the belts that you are replacing.  When the belt is installed and the roller brush is spinning, it is causing the belts to stretch out which will inevitably cause them to break.  The new belts will need to be stretched as tight as possible during installation for them to fit and work properly.  Our suggestion is to use a flat head screwdriver and pry the belt onto the pulley.