The SodaStream system is designed to allow users to choose how sweet and fizzy they want their drinks. If you have used the recommended amount of syrup in your drink, try adding a little more until you find the level of flavor you like best.
Flat soda may indicate that you have not carbonated properly. After twisting the carbonating bottle into place, press the carbonating button ALL THE WAY DOWN in firm short bursts. You will hear a hissing sound at first. Continue to press the button in firm short bursts until you hear a LOUD BUZZ. The loud buzz indicates the first level of carbonation. Three loud buzzes will achieve average carbonation.
Usually this is a sign that there may be two sealer washers in the carbonating gasket head. The sealer washer is a small, black, O-shaped ring about ½" in diameter. To try and correct this problem, first remove the carbonator cover in back of the machine. Unscrew the carbonator from the fitting and lift the machine so you can see inside the gasket head. Using tweezers or a small screwdriver, attempt to remove the black rubber washer from the head. If there is a second one underneath it, you've just solved your problem!
There are a number of indications that show your carbonator is empty:
Not at all. Normally after carbonating a bottle of water, there is a small amount of gas in the soda maker. The tilt of the bottle releases that gas as a safety measure. The amount of gas lost is not problematic, as this gas has already passed through the water as a part of the normal carbonation process.
Make sure your carbonating bottle is filled just to the "Water Fill Line" indicator (the lower of the two pronounced ridges near the neck of the bottle.)
Other possible causes for overflow: