MLSS Concentration dropping in WWTP

Our integrated wastewater treatment plant is now facing the problem low Mixed Liquor Suspended Solids (MLSS) concentration in the aerobic system. The overall performance - removing Chemical Oxygen Demand (COD) efficiency and Total Suspended Solids (TSS) are normal, but the MLSS concentration in aerobic system keeps dropping. We even increased Return Activated Sludge (RAS) to try to build back the sludge contain but did not have success.  How do we stop MLSS from dropping and why is it happening in the process? Has anyone had similar experiences? The wastewater source is petrol chemical industry wastewater.