The most probable cause is that instead of being converted into methane, the substrate COD is converted into carbon dioxide due to presence of oxygen.

To answer this, a physical and chemical analysis of input feed is required. However, I suggest you to examine the chlorine content of your input material, as it is a strong prohibitory element in bacterial activities usually significantly declining the methane production rate.

What is the percentage of methane and Carbon dioxied ?

Because COD is converted to carbon dioxide because to presence of o2