This problem could have a number of different causes. Unless it is a machine specifically designed to operate at reduces water levels, the likely cause is that the sensor that determines how much the machine fills up is somewhat blocked or going bad. The blockage would prevent accurate sensing of the water level. See the explanation below:
The level sensor
uses a pressure switch
to detect the water level in the tub.
Water level control switch
This switch controls how high the tub fills with water.
Water level control switch plumbing
The big end of the hose connects to the bottom of the tub, while the small end connects to the switch. As the water level in the tub rises, water rises in the hose also; but the air in the hose is trapped, so as the water rises, the air is compressed.
Inside the water level control switch
Inside the housing of this switch is a little piston
. The pressure in the hose pushes the piston up. When it is raised far enough, it pops up and closes an electrical contact. This set point, where the contact is lost, is adjustable, and in the picture you can see the cam mechanism
that is connected to the adjuster knob on the control panel of the washer. As the cam turns, it presses a spring against the cylinder
, making it harder for the cylinder to pop up. This means that the water level will have to rise some more before the pressure in the hose will be high enough to trigger the switch.