Some time ago I had noticed that my signature line is added or omitted in what looks to me like a random manner. In the old user interface you could select "advanced" and then could select or unselect a "add signature line" tag. I have not found a similar possibility in the new layout, even if I select the underlined A for advanced. I had also noticed, as Eddie stated, that the signature line suddenly appears the next day. It's not a real show stopper but somewhat annoying. Hope some guru turns up to help us....;-)
(PS: wonder whether this post will appear with or without signature line)