SodaStream Source Instruction Manual page 12

Bottle may not be properly attached. Allow the Soda Maker to finish the carbonation
process and then you can detach the bottle and reconnect it, following the
instructions in the "Make Sparkling Water" section.
It is normal to hear a small amount of gas escape at the end of the carbonating
process.
WANT TO RE-CARBONATE A DRINK THAT WAS ALREADY
CARBONATED:
WARNING: FOR YOUR SAFETY: NEVER carbonate again after you add the syrup.
If you have not added Sodamix, you may recarbonate only water, as explained in
"Make Sparkling Water" section.
WARNING: DO NOT REMOVE THE CYLINDER OR THE BOTTLE. LEAVE THE SODA
MAKER IN AN UPRIGHT POSITION. DO NOT TOUCH IT UNTIL YOU NO LONGER HEAR
GAS ESCAPING. When the Soda Maker is no longer releasing gas, remove the
carbonating bottle, and contact your local SodaStream customer service.