I have noticed that 12864 displays have a variety of options, determined by the presence or absence of components (including 0 ohm resistors) fitted to the board. There are some threads about this on the Displays section of the Arduino forum.
If the pot was fitted but not working, this suggests to me that there is a missing resistor on your board. Try following the PCB traces from the pot to see what they connect to. Posting a close up photo of the back of the display may get you some more help.
If the pot was fitted but not working, this suggests to me that there is a missing resistor on your board. Try following the PCB traces from the pot to see what they connect to. Posting a close up photo of the back of the display may get you some more help.