P&B shakes are great when you are eating food but when MCing, they draw a lot of water out of you along with toxins. Since the lemon in the lemonade is a diuretic, we already have to work hard to stay hydrated. If you don't have enough water in your system, you won't get the P&B shake out the other side, per say.
Some swear by doing the shakes while fasting but it is really hard on your system. I suggest instead you drink lots of water, adjust the salt in your SWF (Salt-Water-Flush) until it works for you, and steep the lax tea longer if you still aren't getting at least 2-3 BMs a day.