P&B shakes make me constipated too! Just taking P & B shakes alone will not cleanse your intestines. I feel you either need to go on a special diet or a juice fast in addition to the P & B shakes. Plus you need to get your bowels moving. You could take enemas, laxative teas, salt water flushes, colonics, or Epson salts.
You need a minimum of one good BM per day, two or three would be preferable.
Also do not stay on P & B shakes for a long time. I believe in 5 to 14 days bowel cleanse, no longer. Take a break so that your body goes back to your normal functioning and start again after one month of rest.
For more info go to my Blog under M for: “My challenge by Propaul”