You really should have started a new thread since this one is more than two years old.
Typically when you vary the pot so that the VO voltage is higher than ideal the display will be very light (invisible) and when the voltage is lower than ideal the display will be very dark (maybe even just two rows of solid blocks).
If the display actually requires a negative VO then the display will still be too light with 0 volts applied (with the pin grounded) so I don't think that is your problem.
My guess is that your potentiometer was defective or incorrectly connected.
Don