With the field in design view, right click the field. Select properties.
Under "Control Source", select the field you want the text box linked to. It's a good idea to give the Text Box name the same name as the field name.
It sounds to me as though your predecessor may have gone about entering data another way.
Again, with the form in design view, right click a text box. This time select "Build Event" and then "Code Builder".
If you see coding such as:
[whatever] = [whoever]
anywhere in the code, your predecessor used a different method of entering data and you have got problems.