When you edit an existing segment that uses constructions, the previously selected construction for that segment on each pole may not be reselected when you go to the "Attachment height" window. Constructions in the dropdown are sorted by attachment height and then the first one in the list is automatically selected. You may need to reselect the desired constriction before proceeding.
This is a known issue in version 6 and will not be rectified at this time. This issue does not occur in version 7.