LM311P never going low
You should use a circuit like this one from the datasheet: 



You can use +5 for Vcc+ but Vcc- should be -12.5V or a bit more negative. 
. The LM311 will work down to the negative supply rail plus a few hundred mV and to within a couple volts of the positive supply rail. 



The emitter connected to ground will give you the 0V/5V output, or you can connect it to -12.5V ~-15V if you want more swing at the output. Adjust your hysteresis resistors correspondingly, and possibly add another resistor to a supply voltage or other reference if you need to adjust the center of the deadband. Also keep in mind that your pot does not have zero output resistance so the width of the deadband will vary as you turn the pot (maximum at mid-scale, minimum at either end).