i wonder though if you wouldn't be able to tell if the es worked or not. did it clean out your system? if so then perhaps the different effect is simply that the results can vary widely btwn flushes according to a lot of postings here.
my Epsom Salts was left in a ziplock bag on the floor of the car for 7 or 8 months in a very hot part of the world and they still do the job. well, i haven't had much luck flushing stones the last few times but i get plenty of butt pee. =) best of luck on your next one.