This occurs in rare cases. This may be due to an air lock between the upper and lower chambers. If there is not enough air between the two chambers, the water will not flow out of the tap.
The first thing to check is the presence of water at the junction of the tanks, due to a filling whose level has risen too high (once can be enough!). You must then wipe the perimeter of the bottom of the upper tank and the inside of the top of the lower tank, and be careful with the next fillings.
If your system was delivered with the metal clip contained in the starter pack, install it between the two tanks and the problem will be solved.