I don't use PP, but some research suggests that you may have some hidden Slides.

Slides don't have a visible Property, but Shapes do.
Something like this may make all Slides visible. : 
Sub Trying()
Dim Sld As Object
Dim Shp As Object

For Each Sld in Slides
For Each Shp in Sld.Shapes
Shp.Visible = msoTrue
Next
next
End sub