Moisture buildup in your flute occurs when your breath condenses inside the flute and clogs the air passage. This is very common, especially if the ambient air temperature is cool or cold, or if you are out of practice. After playing a while the muscles in your mouth adapt, and the amount of moisture going into the flute decreases.
When the flute starts to sound "off," place a finger part way over the sound hole (the rectangular hole in front of the fetish) and blow hard into the mouth piece. Then hold the flute by the bottom end (opposite of the mouthpiece) and shake out any moisture that may have collected inside. This clears out the moisture and you will be able to play for awhile longer.
It also helps to create an embouchure when you are playing the flute. This is described in the instruction booklet on page 7 under "Techniques." The natural tendency is to place the mouth end of the flute into our mouth and blow. Unfortunately that creates excess moisture in the instrument and it limits the techniques one can use to play. Instead, close the lips, then place the flute against them. Allow the upper lip to close a portion of the air hole (half or more) and place the lower lip just slightly underneath the flute. In this way a smaller air hole is created and the air passage from the mouth is reduced as well. This greatly reduces moisture build up and allows for better tonguing and effects control.
Even with better mouth and breath control, moisture is an ongoing challenge and we are continually working on it. We have come up with a technique that helps moisture proof the airflow zone.
At Home Depot we have found a "Brush On" Super Glue in their paint department/glue section. Untie the fetish and remove it from the flute, then brush on the Super Glue to the bottom of the fetish in the air channel - NOT ALONG THE EDGES OF THE BOTTOM (you are not gluing the fetish to the flute). Then brush glue on the flute in the area between the rectangular sound hole and the air hole directly behind it. MAKE SURE TO LET THE GLUE DRY COMPLETELY (about 15 minutes) then remount the fetish.
Super Glue is a liquid plastic and creates a moisture proof barrier. This will help with the moisture but will not completely eliminate it.