What do you mean by every time you fire it? Each time you pull the trigger, or each time you load a cartridge for a shooting session?
The seal you are referring may not be the problem. Inspect it with good light and a magnifying glass. If the seal looks good, and a new cartridge leaks after piercing, it is probably debri on the valve or valve stem face. Putting a balloon on the end of the barrel will help determine where it's leaking from. Keep the bolt closed after piercing a new cartridge and place the balloon over the end of the barrel when the gun is leaking. If the CO2 inflates the balloon, it is most likely debri holding the valve open.