Anytime you pass fecal matter in dry clumps, hard compact pieces or pellets...it's an indication that you are dehydrated. The way to keep the colon well hydrated is to keep the entire body well hydrated...water is pulled from the chyme (liquid digested material) as it passes through the transverse and descending colon...and your colon will continue to pull that water as long as the body needs it.
In addition to drinking sufficient water...and watching your intake of foods and activities that contribute to dehydration such as caffeine, alcohol, frequent flying, strenuous activity, etc....a series of colonics can help to rehydrate the bowel...and hence the body. They also help improve peristalsis so waste moves through faster...fiber also does this.
Avoid the senna...and any other laxatives...they only make the problem worse over time.
Have you tried drinking a couple of glasses of warm water every morning? Are you getting regular exercise? Do you take minerals regularly? A build-up of mucoid plaque can also inhibit normal peristalsis...and a 3-week Colon Cleanse is most likely not long enough to remove most of it. Stress has one of the most powerful influences on digestive functioning... if you do everything else 'right', but don't manage stress well...this alone can cause all sorts of digestive problems.
As for the parasite cleanse you are doing...frankly I couldn't tell you if it will do the job...I've only used pre-dosed parasite combination formulas...never individual herbs. Someone on the parasite forum can probably tell you, though. ; )