If your SodaStream Source is producing flat soda or soda without enough bubbles, try the following solutions (roughly in order from easiest to hardest).
Water Temperature is Incorrect
When using the SodaStream Source to make soda, using warm water generally results in fewer bubbles. Cold water is recommended for best results and more bubbles. Clean out the carbonation bottle and refill it with cold, unflavored water, and try to use the machine again.
Did Not Start with Carbonated Water
For fizzier soda, SodaStream recommends using unflavored carbonated water. The soda will have more overall fizz because you are adding more carbonation, to begin with. You should not be using juice, flat soda, or any other previously flavored liquids. Clean out the carbonation bottle and refill it with clean, unflavored, carbonated water, then try to use the machine again.
Carbon Dioxide Cylinder Needs to be Replaced
If your SodaStream is producing soda with little carbonation, replace the carbon dioxide cylinder with a full one.