Hi Cathy,
Sounds like we need a more cooling action on the colon with the herbs, given the benefits of metronidazole. You can try switching XCHT for Da Chai Hu Tang, or even using Chai Hu Jia Long Gu Mu Li Tang, if you could call the pulse slippery and/or superficial at all.
If that is the case, Yang and Blood are trapped in the exterior. In that case, we should not use Blood movers (which drive Blood peripherally). Instead, we should use something like Hoxsey. Response to acupuncture points can help a lot in deciding whether this is a good idea.
But if the pulse is definitely deep, then increasing the dose is perhaps the best approach. I always tend to dose more aggressively in a dog that is not adequately controlled. In a 16 lb dog using Kan tablets, you're looking at as many as 5 x 550mg tablets to achieve the same dose as if you were using granules. So you have a lot of room to go up, I think
S