If there are no other signs, my 'old reliable' method of stimulating colon motility is Cascara Combination. You can get this from Mona. It's quite effective. It doesn't taste great, but you can encapsulate it, dilute it, or even gently heat it to get rid of the alcohol. Just 0.2 ml per 5 lbs should work, used as needed, up to TID. Another option is to bump up the dose of the Ma Zi Ren Wan. Tea pills are quite weak and not well utilized by dogs and cats compared to liquids or granules. Increasing the dose may compensate for that. Lastly, if there are vertebral fixations in the lumbosacral region, they'll impinge on the sacral plexus and reduce the strength of nerve signal transmission to the colon. Chiropractic would be able to detect and relieve any fixations that are present.
Hope that helps!
Steve