There is a thing with the geothermal generator's GUI where you put in your lava cell and the generator accepts it and outputs power but the GUI won't show how much lava's left to burn. The exterior does change to show it's burning but the inside doesn't change at all and I can't tell if I need another cell yet.
Are you talking the bar on the right or the square in the middle? If you're talking about the square in the middle, I've found I need to put in at least 3 or 4 buckets of lava before that square will show any change.I've never seen the bar on the right change ever, though.