DIAGNOSIS: The water filter is clogged and overdue for a change, or there's a kink in the water line behind the fridge.
FIX: Replace the water filter. Twist the old one counterclockwise in its housing (have a towel ready for drips), and insert the new one, twisting until it clicks. Run water for 3-5 minutes to purge air.
IF THAT DIDN'T WORK: Check the water supply line valve behind the fridge. Make sure it's fully open. Also, inspect the small water inlet valve at the back-bottom of the fridge; its screen can get clogged with sediment.
HEADS UP: Using generic or off-brand filters can cause this. LG filters have a specific flow rate. If you didn't buy an official LG filter, that's likely the culprit.