Do you have any objects that touch or cross the line? Try holding down Alt (don't know windows equivalent for this one) and dragging to see if you can reposition the line.
If an object touches or crosses the boundary between header and body, it will be treated as though it were part of the header.
Also check for invisible objects that may still be there. Do a select all (Ctrl-A) and look for selection handles.
And actually objects don't need to touch or cross the line if they exist in the header; they will not slide up when you reduce the header size. Objects in the header must be moved up before reducing the size. Otherwise as suggested, look for objects in following parts that are too high and look for invisible objects.
One of the most "invisible" objects tends to be the label. Sometimes the default text color for labels gets set to your background color. This can drive you nuts. Also make sure that there isnt simply a "locked" line or other object sitting on the line either. (I dont think you can lock the "header" size but its something to look at too)
thanks all. I had some objects that did not appear to be in the way, but were. Fixed. Thanks!