Recently bought house with reverse osmosis water filter unit under kitchen sink. No instruction manual. Have a large plastic "key" apparently intended to remove the bottom of filter "cans" (3), but am unable to loosen them. Are they possibly reverse threaded?
Perhaps you need to move the key up so it fits tight before you turn it. I had a similar problem before and like the previous message, turn it hard but carefully. Hopefully it's not because the previous owner of the house did not change the filter for a long time and there is now excessive dirt along the fittings.