DIAGNOSIS The water inlet valve screens are clogged with mineral deposits or sediment from your home's water supply.
FIX Unplug the washer and turn off the hot and cold water supply valves at the wall. Disconnect the fill hoses from the back of the machine. Use needle-nose pliers to gently pull out the small mesh screens from the valve inlets. Rinse them clean and reinstall.
IF THAT DIDN't WORK The water pressure from your house might be too low. Check that both supply valves are fully open and that the hoses aren't kinked.
HEADS UP This is maintenance. Do this cleaning once a year if you have hard water to prevent slow fill times and potential valve failure.