This is purely anecdotal, ...

This is purely anecdotal, however, we have one RO bank where some unknown "foulant" apparently got on the membrane surface, both 1st permeate pass, and 2nd permeate pass.  The transmembrane pressure went up by minimum of 60%, but oddly no increase in the differential pressure across vessels.  Salt rejection also increased (which seems counterintuitive).   For a water supply with reactive silica in the 20 ppm range, and approximately 1600 microsiemen/cm conductivity, the second pass permeate is normally below 2 conductivity, and the silica is less than 10 ppb.  Alkaline surfactant cleanings did nothing to remove the foulant at all.  Although the feed pressure is now near the vessel limit on the 2nd pass, we are at least able to operate the unit, and the water quality is surpassing.

Any ideas?  I had thought perhaps this was oil fouling, but how would that stay put indefinitely on the membranes?