Because of the "sleep" feature I added, my contrast is actually PWM (pulse width modulation) controlled. When about 10 seconds pass the screen's contrast goes to minimum and the backlight shuts off. Making it look as close as possible to the rest of the case. so I actually use a far different contast control than just about everyone.
If you are using a serial backpack module you'll probably need to find a slightly more powerful contrast pot. First thing is to meter your input voltage going to your contrast line on the LCD and make sure you are getting a reading that is within your LCD's spec.
Some newer LCD's even have a digital control built in (special data line instructions are needed to control this) and the analog is some what of a "limiting" knob, that way someone can set that and never make their display unreadable due to a software bug.