The level sender is a variable resistor that is wiped up and down by the float arm. If it is reading a fixed resistance regardless of where the arm is, then either it is;
Shorted close to one end of the winding,
Really dirty in some way (although this usually results in high resistance, not low),
Wiper broken off the arm so it is stuck in the one position,
Any of a variety of other combinations of the above or something outright surprising.
Play with it. Work out what it is doing wrong.