Why is my O-ring leaking?

If you have the O-Ring seal with you, Does the seal appear damaged, deformed, swollen or physically changed in any way? If YES, then please refere to http://www.phelpsgaskets.com/blog/troubleshooting-o-ring-failures If NO, then does the used O-ring cross-section still meet the original print dimensions? If YES, then most likely causes of leakage: 1. The O-ring is not being squeezed enough to ensure reliable sealing. 2. The surface finish of the mating parts is too rough to attain a good seal, or the metal surface is scratched. 3. The application is exceeding the low temperature capabilities of the material. 4. Gas is permeating through the seal material. 5. Explosive decompression could have resulted in the internal splits in the material. If No, then the most likely causes of leakage: 1. The seal could has shrunk. 2. The incorrect size O-ring was installed, so the amount of squeeze is insufficient to ensure reliable sealing.