Constipated from Vegan diet!

Hi.

My husband and I will be starting our first Liver Flush soon with a series of colon hydrotherapy sessions starting soon too. To prepare, we have been eating vegan for just over a week now (not only raw though), and my husband has become constipated. He has always been a 3 a day man when eating meat and even junk food, and now that he is eating clean vegan food he can't go! Why would this happen? Any thoughts or knowledge you can share?

Thanks!