I've also been growing potatoes in bags and continually covering up the new green shoots as they poke through. My shoots are now three feet above the top of the bag and looking very lanky so it won't be long before they start slowering and then die back. It's important to check near the surface of the compost to ensure that any top potatoes are not exposed to the sunlight and go green, otherwise you'll have to disgard them. Keep the compost well watered and if necessary cover any exposed potatoes with scrumpled up moist newspaper to protect them from light.