I've had this problems. The issue with wood is there there are several types. The standard 'bogood' is most often actually mopani wood, which is a tree from Africa that they cut into pieces and sandblast. This will have little effect on the tank. Then there is the actual bogwood, which is wood taken from bogs, etc. This in my experience anyway reduces KH. I dont understand the science, but would think it's probably a result of that fact that some bogs (if not all?) are acidic my nature, anything that sits in them for years will also become acidic. Put that in your tank and the acid will use up the KH.
I had exactly the same problem. I did my usual KH test and it was 0, it was only after removing the piece of true bogwood that I could get the KH level to be stable.
Sam
PS - there is also grape vine wood, redmoor wood and petrified wood, I've not used any of these so cant comment on what they would do.