Was the refrigerator just installed?
Depending on the model, it can take up to twenty four (24) hours for the water to properly chill after installation.
Was a large amount of water just dispensed?
If a large amount of water was just dispensed, it will deplete the water reservoir. The new water needs time to chill. Allow time for the new water to properly chill (time depends on model).
Is the dispenser being used regularly?
If water is not being used very often the first glass of water may not be as chilled. Discard the first glass of water.
Is the refrigerator connected to a cold water supply line?
Verify the unit has been hooked up to the cold water line. Installing the refrigerator up to the hot water line will cause the unit to dispense warm/hot water.