Does your hydration system have a shutoff valve? If so, it may be closed. Open it by twisting the bite valve 90 degrees (Clockwise closes it, Counter-clockwise opens it).
If that doesn’t work, you may be biting the valve too hard. The Platy HyperFlow Bite Valve is designed so that you don’t need to bite very hard. If you bite too hard the flow will be completely cut off. With your teeth, compress the valve lightly (only 30% to 50%) and water should flow.