SWF won't work if you have a lot of waste in the bowels. Perhaps places that are impacted, and then the water pushed some wasted to that point and doesn't have the strenght to push thru. I had the same problem, i would usually have to do enemas for a few days while on master cleanse, before the SWF (Salt-Water-Flush) would go thru. Otherwise, it would just slush around in the bowels whole day. That really is the main concern, no matter how dehydrated you might be, something would be coming thru if the paths were open.
Ofcourse if you're just trying SWF (Salt-Water-Flush) to have a bowel movement while not on some type of a cleane, that's rearely a good idea as it will get stuck more often then not, partifulyrly if you have dry stools.