The prep will begin working eventually. It can sometimes take a couple of hours to start having an effect, and in patients with constipation, it can take longer. Adding a couple of glasses of water to your prep, in addition to the standard instructions, and taking Dulcolax/bisacodyl tablets at the beginning of the prep will help things start more quickly. Only contact the on-call physician after the second dose of the prep if you still do not think it is working well enough.