I know from reading on the Whole Approach forum that constipation isn't that uncommon when you start with the psyllium and Bentonite shakes. If you add Caprol to your shake it helps lubricate and also helps with the cleansing. It made a big difference for me. I went from going every other day to going twice a day. I agree with Richard that taking a good probiotic with bifido bacteria also helps.
I can't remember the reason for the constipation when you start but I know it has been discussed there.
Don't give up. It gets better.