STPP has one weakness, and that is that over time, with exposure to water, it will decompose into a mono-phosphate, or "orthophosphate", called trisodiumphosphate, or TSP. TSP is often used for cleaning hard surfaces where a precipitate is not a problem, but due to its precipitate formation is not favored for laundry use. "
TSP also works, but from what I understand is tricky to handle (more of an irritant) and can precipitate as describe above. Could be that you're using yours up before the whole batch goes changes -- I just don't know what the rate of degradation is.