You have to be well hydrated, the epson salts will draw out every ounce of spare water in your system, then expell them through your bowel.
If the water isn't there, nothing is coming out and in the coming days you may add water weight, the fecal matter will absorb the epson salts, and days, weeks, or months later the epson salts will come out in your bowel movements.
drinking water is extremely important. I personnaly drink water right up to the olive oil/grapefruit drink. Don't drink it after!