The most obvious cause of "dry bowels" IS dehydration. That is just plain common sense. Then you are ignoring the recommendation to do the lax tea in the previous evening "to loosen things up". Then there is the recommendation of the morning protocol of drinking 16 oz of water upon arising and one qt of plain water if no movement has happened within an hour of doing the SWF. In addition there in the FAQS of the MC forum there is a description from an Ayurdvevic doctor
of some exercises to do along with the SWF to help it work. BTW the SWF is not unique to the MC. It is a very old Ayurvedic protocol.
If you keep having to rely on enemas to make the SWF work for you, the other obvious thing is that you have a problem with your bowels that you need to address. It is not recommended to do the MC if you are not having at the very least one unassisted bowel movement each day.