make a quick test file
see what this is going to do. if its what you want, then try it in your original

– Right Click on your sheet1 tab and choose View code (or just hit Alt F11)
– cut and paste this

Sub SentenceCase()
 For Each cell In Selection.Cells
 s = cell.Value
 Start = True
 For i = 1 To Len(s)
 ch = Mid(s, i, 1)
 Select Case ch
 Case "."
 Start = True
 Case "?"
 Start = True
 Case "a" To "z"
 If Start Then ch = UCase(ch): Start = False
 Case "A" To "Z"
 If Start Then Start = False Else ch = LCase(ch)
 End Select
 Mid(s, i, 1) = ch
 Next
 cell.Value = s
 Next
 End Sub

– then highlight your column (or whatever you want to change)
– then run the macro
– click tools
– click macro
– choose sentance case and run