Often, we get calls about a leaking CO2 system. It is very rare that a component is failing to cause this leak. Under most circumstances, reviewing the installation and set up of the CO2 components will resolve you issue. You want to make sure you have the washer installed at the tank side of the regulator and the duck bill washer (or one way check valve) is installed at the coupler. If one of those are not present, you will have a CO2 leak. You may also have a loose clamp or one of the nuts to tighten the components are loose. Click on the link and review the video below for a better visual.
If you are confident that all of the pieces are installed in the correct place, the best way to find a CO2 leak is by spraying soapy water on all of the connections. Anywhere a CO2 line has a clamp, or a nut going on to another part is where you want to start. A mix of a little bit of soap and water sprayed from a squirt bottle will bubble up and help you narrow down the leak.
You can also submerge the CO2 line and coupler in a bucket of water and look for bubbles.
Here is a video we found on YouTube that goes over some common things to check.