On a juice fast one is going to have less movements a day. the thing is to keep the Colon moving. Use your intuition, listen to your body... do you have a head ache, if so then you might be getting plugged up. If not just keep the positive diet going.
If you have had a history of Colon issues, you might be cleansing all the packets and diverticui in your Colon. If this is the case you will want to take something that helps push that through since the lack of weight from the food is missing for your "normal" movements.
I have found that Dr. Richard Schulze Intestinal formula #1 is a great way to get things flowing... even on a juice diet. But don't over do it like I did and have the runs... It basically comes down to tuning in and making your cleanse an art form.
I hope this helps,