I must admit up front that I have a strong bias against open-system colonics. I've had and administered colonics on a Libbee...didn't think much of it. It was not nearly as effective as the closed-system colonics I have had...nor was it particularly comfortable...more like using the loo in front of someone. I'd check out the place in Houston if I was you.
If you are not eliminating while doing the P&B shakes...stop them! You will become more impacted and it will be harder to clear out the material when you get the colonics. I would also suggest that you do some plain-water (purified or distilled) enemas at home to get things moving again. Once you're going again...you can resume the P&B...but you must keep things moving!
The two things that can make a huge difference in your colonics...being well-hydrated and relaxed! Make sure to really slam the water between now and your appointments...and try to relax as much as possible. Even little things...like allowing enough time to get to your appointment without driving in crazy traffic...can make a tremendous difference in how well you release during the colonics. We hold a lot of stress in our gut! Become very conscious of your breathing during the colonics...and try to breathe as deeply, but comfortably, as possible. Also...lay off the fries and ice cream...and avoid any flour products...breads, crackers, pastas, cakes, cookies, pretzels, etc....they will just clog you up all the more.
You can also look for a referral for a colon hydrotherapist at the following link. But if this place in Houston has a good reputation...I'd go there.