Poor Settling in Wastewater Digesters

Our facility is a 2 MGD Sequential Batch Reactor Activated Sludge Wastewater Treatment Facility. We waste our solids to a sludge anoxic holding tank from the SBR tanks when decanting. When the sludge anoxic tank level reaches the set point, it begins to feed one of our digesters. When the digester reaches a high level, it begins to feed back into the sludge anoxic and it continuously circulates between the two tanks until we isolate the digester and cut off the air to decant the clear. It has operated that way for years but now it is not settling well or at all. There are occasions of popup sludge or no clear. Our SBR profile has not changed and nothing in our lab testing has shown any changes.  What could be causing poor settling and can we solve it?