Check your Coagulant dose ...

Check your Coagulant dose rate seasonally to determine the optimum dose for each season because any change in raw water quality could results in flocs not flocculating and settling within the stipulated time.

In this case you can use a flocculation aid or Bentonite Clay to make up for any of the decrease in the + or - charges otherwise called electrostatic forces of repulsion and vanderwaals forces of attraction.

Another possibility is the raw water Alkalinity, if for any reason the dissolved alkalinity is low and you're dosing Hydrated Lime to raise the Alkalinity, there is need to check that because it can create a secondary precipitation that could be carried over to the filters as well.