You may have a fault in the line. For example in one of the junction boxes. I had this exact situation. I tried many things including verifying that I had the wiring done correctly in the junction boxes and also changing the junction boxes. Nothing helped, until a B&O tech visting the house for another reason told me that the "only" solution was to hardwire the ML nodes with some wiring crimping devices and eliminate the juction box in question. I had my doubts, but he insisted. He was right. the volume on that ML node now works perfectly.
I should not that this is the only node where this was done. All others are wired with junction boxes.