I'm running my boiler without storage, lots of creosote, but this is my fourth year with it, and this problem is new this year. Sometimes I have to bang on the damper handle with my hand four or five times before it releases. I cleaned the damper over the summer when I cleaned the heat exchange tubes, and I can get it pretty clean any time I let the fire go out with a long screwdriver by reaching into the firebox and scraping through the grate. But it starts sticking again after a day or two. Nothing's different this year from last year. I'm now burning the wood I didn't get to in the spring, nice dry oak mostly. Any ideas why it would start sticking now?