Pure ammonium sulphate should still be usable, though you may have lost a bit of it to the floor, or wherever the water ran! The problem lies with blends, which will change formula, due to differential solution of the different elements. Organic fertilizers change as they decay with moisture, most often losing nitrogen to de-nitrifying bacteria. Those change the fixed nitrogen--i.e., proteins, uric acid, urea, ammonia, etc.--to gaseous nitrogen, which floats away.