Sometimes authors will add Content Controls and then lock them. This means that certain parts of the document can't be changed and it's not obvious why. Also it's not possible to go through and unlock these without knowing exactly where each content control is - it could be in a header, it could be in a frame, etc.
The following code will try to unlock each Content Control it can find - it seems to work on headers and text boxes, but perhaps there could be others it doesn't do.
Sub UnlockAllParenContentControls()
Dim aCC As ContentControl
Dim aRng As Range
Dim shp As Shape
Dim frmRng As Range
' Unlock content controls in main story ranges
For Each aRng In ActiveDocument.StoryRanges
Do
For Each aCC In aRng.ContentControls
aCC.LockContentControl = False
aCC.LockContents = False
Next aCC
Set aRng = aRng.NextStoryRange
Loop Until aRng Is Nothing
Next
' Unlock content controls in shapes (e.g., text boxes)
For Each shp In ActiveDocument.Shapes
If shp.Type = msoTextBox Then
If Not shp.TextFrame Is Nothing Then
If shp.TextFrame.HasText Then
Set frmRng = shp.TextFrame.TextRange
For Each aCC In frmRng.ContentControls
aCC.LockContentControl = False
aCC.LockContents = False
Next aCC
End If
End If
End If
Next shp
End Sub