This is very extreme, but you may want to try the colonoscopy prep routine if you can't find anything that works, and you're determined to get cleaned out.
In my case, I was instructed to buy a 20-pack box of Miralax powder, and dissolve 1 packet in 8oz (235ml) of water and drink it every 15 minutes the day before my colonoscopy. 16 packets in all. After the 4 hours of Miralax every 15 minutes, follow-up with 4 tablets of Dulcolax. After the first 3 or 4 rounds of Miralax, you'll find yourself on the toilet every 15 minutes, corresponding to each successive round. As I said, this is very extreme but your colon will be clean as a whistle. Miralax is a form of polyethyeneglycol (a very close cousin of antifreeze), so I'd do this only if absolutely necessary. And no, I'm not an MD.