Presumably because whatever Split(StrDetails, "|")(0) contains - and populates "A-OFFICE" with - is neither "OFFICE01" nor "OFFICE02".
Based on what you've posted, your code would be better expressed as:
Code:
With ActiveDocument.SelectContentControlsByTitle("A-OFFICE")(1)
.LockContents = False
.Range.Text = Split(StrDetails, "|")(0)
.LockContents = False
End With
With ActiveDocument.SelectContentControlsByTitle("OFFICEHEAD")(1)
.LockContents = False
Select Case Split(StrDetails, "|")(0)
Case "OFFICE01": .Range.Text = "OFFICE01 HEADER"
Case "OFFICE02": .Range.Text = "OFFICE02 HEADER"
Case Else: .Range.Text = "JUST HEADER"
End Select
.LockContents = True
End With