Olivia, when magnesium (Epsom) is not having a cleansing effect, it often means you are magnesium deficient and the body just takes it in, nothing left in the intestines.
I believe you need to supplement Mg until laxative effect shows up; and maybe then continue with the laxative effect until you wiped-clean the intestines from top to bottom. Another option is to have 1 lbs of greens every day - salad or juiced.