I'm having trouble removing my chimney cap to clean off some creosote buildup. I had the upper section of the chimney replaced last summer, and the new cap has a different design than my old one (which was held on with a metal band and a single screw). I think the new cap is made by Dura. The installer says it's a twist-lock design and I don't need to remove the screws at the base of the cap's "legs," but I can't get the cap to budge when I try to twist the whole thing off. What am I doing wrong?