Hidden Header

I have a document that contains a top header, bottom footer and a left margin
header. I have used this form for several years. When I need to update the
left margin header information, I would open the form with all information
viewable, click on View, then Headers & Footers. From there I was able to
click into the left margin header and update information as needed.
Recently, when I open the document, I only can view the header and footer.
The left margin header is not viewable until I click to preview the document.
From preview, I cannot update the information. Any answers on what has
happened to my ability to see the left margin information upon opening the
document and how to correct this? My thanks.