The rubber washer on the end of the stem mates or 'seats' on a fixture in the valve appropriately called the seat. They are often replaceable. It's likely that those are bad, and it may have damaged the new ones. Assuming they are removeable, you need a special (inexpensive) wrench to take them out. Since the old seats are likely cracked and pitted, they've likely eaten up the new washers on the new stems.