Was the bottle overfilled?
Make sure the carbonating bottle is filled just to the "Water Fill Line" indicator (the lower of the two pronounced ridges near the neck of the bottle.)
Was only water being carbonated?
Only carbonate water. Don't add soda mix to the bottle before attempting to carbonate.
Was the level held down?
Make sure to carbonate using short firm presses. If you hold down the carbonating button/lever, this may cause leakage.
Is the bottle damaged?
Ensure the carbonating bottle is not deformed in any way. This may occur due to carbonating bottles being exposed to extreme heat i.e., dishwasher.