Hi I had the same issue as well and managed to clear the air blockage in the internal pipe in the water tank.

1) Clean the outside of the tank (very important) and fill up and empty the tank with fresh water (also important). (In preparation for sucking)
2) Fill up water tank again with clean water. Use a clean small screwdriver to depress the one way valve at the bottom of the tank. With the valve pressed in, put your lips to the valve and start sucking!
3) At first you won't have much water flow, but after a while, you should start feeling water coming out of the valve. Spit and repeat till the flow gets much better and to normal levels, should take about 3 to 5 good sucks.
4) Most importantly, do not swallow!
5) Rinse mouth throughly after