Engine Coolant light

I have a 2000 Olds Alero. I have had some issues with the car lately, but the most recent is that the engine coolant light went on. I checked the level of the coolant and it was low. I took it to have the coolant topped off at the shop, but the light has stayed on. What could cause the light to stay on and not go off when the level of coolant is where it should be? (according the line on the container)