Normally with modern versions you would use the TextFrame2 (and TextRange2) object. This allows you to set different indents for each paragraph.
Since you are trying to get them all the same this might work (using the easier to understand legacy TextFrame / TextRange object)
WORK ON a copy!
Sub bulletMe2()
Dim osld As Slide
Dim oshp As Shape
Dim L As Long
For Each osld In ActivePresentation.Slides
For Each oshp In osld.Shapes
If oshp.HasTextFrame Then
If oshp.TextFrame.HasText Then
With oshp.TextFrame.Ruler.Levels(1)
.FirstMargin = 0
.LeftMargin = 18.5 ' adjust as needed 72 per inch
End With
End If
End If
Next oshp
Next osld
End Sub