Tum tum tippy up cup

Just got a tum tum tippy up cup to try and help my LO with drinking more water. It says its free flow but if I tip it upsidedown no water comes out of the straw, it only comes out if sucked. Is this just the design of the cup or is there something I'm doing wrong or an issue with our cup? Opening and reclosing the lid seems to help the water flow but once the cup is placed upright again or my LO puts it in her mouth it seems to stop the water flowing again. Anyone have one of these cups and can tell me how it's supposed to work? 🙃