epson salts lost effectiveness

It seems that the ES solution no longer causes me immediate bowel movements. The first few flushes, the ES mix will immediately cause BMs. however, after several flushes ive noticed that the BMs came later and later after the initial ES dose. now, at my 15th flush, i can go all night w/o a BM and it will be a few hrs after my morning dose til my 1st BM.

anyone know whats going on? am i weakening my kidneys?