Comparing Three Rescue Bowel Preparation Regimens for Colonoscopy After Failed Cleansing

The goal of this clinical trial is to learn which rescue bowel preparation works best for adults who need a repeat colonoscopy after a previous bowel preparation did not clean the bowel well enough. The study will also look at how satisfied participants are with each preparation. The main questions it aims to answer are: * Which bowel preparation helps more participants achieve adequate bowel cleansing before repeat colonoscopy? * Which bowel preparation leads to better cleansing in different parts of the colon? * Which bowel preparation is better tolerated by participants? Researchers will compare 3 rescue bowel preparation regimens to see which one works best for repeat colonoscopy after a previous cleansing failure. Participants will: * be randomly assigned to 1 of 3 bowel preparation regimens * take one of the following regimens before repeat colonoscopy: * 4 liters of polyethylene glycol (PEG) plus 12 milligrams of bisacodyl * 1 liter of PEG plus ascorbic acid plus 10 milligrams of bisacodyl * 2 liters of PEG plus extra PEG taken over the 3 days before colonoscopy * undergo repeat colonoscopy * rate their satisfaction with the bowel preparation