It's been many years since my chemistry in power plants, but as I recall, silica is a cation and your anion bed won't remove it... your initial <2 ppb concentration will concentrate in the boiler drum which your blowdown is suppose to remove -- I would suspect that by recycling your boiler blowdown through an anion bed instead of a mixed bed (or two-bed), you are re-introducing the silica into your boiler stream... also, not all silica remains suspended, so any cycling, any maintenance activites, any distubance in the system will affect the silica levels in the boiler water (also check for leaks in your sample cooler; I assume you've done all the routine calibrations, etc on your analyzer, checked blank samples, etc...)...