PB Shakes and Constipation Any Suggestions? Herbal Laxatives?

Hi, I have tried to increase my consumption of PB shakes to twice a day. Each time I do this I get so backed up I have to stop until I am regular and then I start at square one again. I really feel like taking PB shakes is the right bowel cleansing method for me. Once my bowels get moving again I pass small ropes and other scary things, and a red rosacea- like spot on my face has faded. I drink 80 oz. of water a day. Should I take more fiber? Is there anything I can add to my shakes to make me less prone to constipation? Thank you for any advice.