1906a656_uide_to_Small_Batch_Composting__POSSIBLE_CAUSE
[SYMPTOM] Compost has a bad odor, like a mixture of rancid butter, vinegar and rotten eggs. [POSSIBLE SOLUTION] Mix in some dry carbon material. Maintain proper moisture control. [POSSIBLE CAUSE] 
Not enough oxygen, or too wet, or too compacted.
How to Compost: Guide to Small Batch Composting
POSSIBLE CAUSE