Are you sure the calf isn't feeding when you're not there? If the cow has been injected with oxytocin the milk should now be flowing easily.
If the calf is sucking and head butting it sounds like she knows what to do . A young calf gets what it wants in a few feeds a day and in between will not show any interest in sucking until it is hungry again. By now, if the calf is feeding, you should see the digested milk appear as yellow droppings - always a good way of telling that all is ok.