I have a 80’s era non cat pre epa federal airtight CDW stove. Rear vented to a clay lined ext chimney (covered in stone). Chimney sweep loves its design and the draft it provides. Just this week suddenly I can’t seem to get it to draft unless I closed all the vents. If I open the vent I. The side or the door it leaks out of the corners where the stove cement is gone. So I have had three fires like this and I don’t know what’s wrong. Now I got a decent coating of creosote inside the stove. Any ideas ?