This problem just cropped up this morning. When I tried to switch the damper to vertical position, it wouldn't stay in place, just falling back to horizontal. I had a hot fire so I couldn't look to see what was going on. The damper seems to go completely vertical, just won't stay that way. The quick peek I took didn't show any obvious blockage. This stove has probably been here for 30 years. I've lived here for 5. I assume that the previous owners never refurbished it. I'm guessing that there is a build up on the edge of the damper that prevents it staying in the full upright position. It's going to be several hours before it cools down enough to check it out. What should I be looking for?