A very common occurrence with CO2 Airguns and Airsoft guns are leaky CO2 seals. Usually this occurs right where the CO2 cartridge connects to the main valve area. This can be either in the gun or in the magazine depending on the setup of the airgun. There are many reasons why this can happen, but the most common reasons are a worn-out or dried up seals or even a seal that has been imprinted or misshapen from the constant pressure of the tip of the CO2 pressing against it. Read full article here: https://www.replicaairguns.com/2014/5/29/how-to-fix-leaking-co2-in-an-airgun.html