My doctor said that my bowel was too full of stool to do a colonoscopy even though I did the bowel prep. How is this possible?

Yes. Unfortunately this can happen. Next time your doctor may suggest a liquid diet for a couple of days prior to the colonoscopy and they may also change the prep solution. Good luck!
Depends. People who tend toward the constipated side may need more than a 1 day prep to adequately perform a colonoscopy.