Why are the laxatives not working?

I have a colonoscopy tomorrow and have followed the regular routine. Up until today, I have been regular..painful, but regular. Today, nothing. Took the prescribed 4 laxative pills, drank about half the miralax crap before I got sick, and then went and bought more laxative and took about 6-8 pills. AND NOTHING. I don't understand! I am having back pain, which I have had since I took the four earlier today. What could it be? I truly don't understand and I really need this test for medical reasons.