Interesting that you have a hard time "tucking" while walking low. What this most likely means is that your lower back muscles are not balanced with your abdominal muscles. For beginners whose legs lack either the muscle or tendon strength to support low walking, there is a tendency to tense up all over. The result here, if I am correct, is that the pull from the abs overrides the counterforce your lower back muscles should be exerting. Also it is possible that your lower back muscles lack the "length" or elasticity to tuck correctly. Either one or both can be corrected with targeted exercises unrelated to bagua.
BTW, if you hold a low horse stance can you keep your spine straight and pelvis properly tucked?