Check your jumper selector switch. If you have a wire in the RC terminal and no wire in the R terminal, move the RC wire to the R terminal and move the jumper selector switch up to the "1 Wire" position. If you have a wire in the R terminal and a wire in the Rc terminal, the jumper selector switch must be in the "2 Wire" position.
Check your wiring. If you have a heat pump, you will not use a wire in the W terminal. Move the W wire to the W2 or Aux terminal. (W controls your first stage of heating for Gas/Oil/Electric systems. O/B and Y control your first stage heating for heat pump systems. Backup heat sources must be connected to the W2, Aux, or E terminal for proper functionality.)
Check your configuration. Most commonly, O/B is energized in Cooling (O on cool). For Rheem or Ruud brand heat pumps, the O/B must energize on heating (B on Heat).