If you are using scripts to manage M3, M4 and M5 and aren't using any of the on board outputs (my spindle is controlled using an I2C board), so under Settings-Spindle all the output pins are set to ---. When you click the spindle toolbar button or run M3/M4 the spindle state doesn't change (ie. not highlighted). When you click the button again rather that running the M5 script it runs the M3 script again.
Cheers
Simon