i would take the sender unit out and see if the bowl is full of gas it might have a whole in it,you can manually test it by taking off the cover and move the float bowl up and down
The sender bowl normally is full of gas - that's why there's a gasket on top.
If the wire comes off the front of the sender rather than the rear, the sender is installed 180 degrees out--the float will not descend all the way into the well at the bottom of the tank. Then if you have a sunk float that might show around 1/4 tank all the time.
If you don't have a sunk float, and if adding a ground at the gauge and at the sender doesn't help, try running a test wire from the post on the sender to the gauge terminal on the gauge, to see if the gauge functions normally.
If you take the "lid" off the sender, be extremely careful of the hair thin wire that goes from the post to the front of the coil. It is super-easy to break.