Try doing a Ctrl-A ( or Mac Equivalent ) to make sure there is nothing hidden.
Under certain circumstances, a layout object can be invisible. You can create a transparent rectangle with a 0 pen width or transparent border for example. Try entering layout mode and choosing "Select All" (ctrl-A in Windows) to see if any selection handles appear down at the bottom of the layout part. If you see some, click an empty space on the layout to release the selected objects and drag a selection box around the area where you Select All revealed the object. Then delete or reposition the selected object so you resize the layout part.
Another possibility is that the layout has become corrupted. If this is the case, you may need to delete this layout and make a new one. Before deleting the old layout, you can copy and paste your fields and other layout objects from the old layout to the new.