The entire desired object or segment must be displayed within the Plan View for it to be selected. Zoom out to show the entire object/segment that is desired for display prior to creating a Skew Section.
Ensure that the desired items are checked in the Show/Hide dialog which can be accessed using the View menu. Refer to Section 6.2 in the User's Guide and Section 2.3.1 in the User's Guide Addendum for more information.
Use Recompute View Extents in the View menu to recompute the active run's coordinate extents. If that doesn't work then there must be an object in the blank-looking area of the Plan View. Use your mouse to select the blank area and see if any objects become highlighted. Then use Delete Entire Object in the Edit menu to delete these objects.