Border weight xlHairline ignored

Border weight xlHairline ignored

In a new workbook any of these lines work
ActiveCell.Borders(xlBottom).Weight = xlHairline
ActiveCell.Borders(xlBottom).Weight = xlThin
ActiveCell.Borders(xlBottom).Weight = xlMedium
ActiveCell.Borders(xlBottom).Weight = xlThick
However, in another workbook with hundreds of macros, only this line does
not work
ActiveCell.Borders(xlBottom).Weight = xlHairline
I have found no reported instances of others having a problem with
xlHairline, and have run out of ideas.