Excel VBA - Stops Selecting Visible Rows on Filter after many interations - sorting

Question

My code takes a file name from TextBox1, opens that file and indexes through all the unique values in column B. It then takes a second file, file name in TextBox2, and filters it based on the current index from the first file. It then takes the filtered results and copies them to a sheet in the new workbook. A new sheet is then generated in the new workbook to paste the next filtered result.
My issue is that I have many rows of data and for some reason the filtered data is not be selected after many iterations. My program selects all filtered data when it starts, but at some point it just begins selecting the headers instead of all the visible cells. Let me know if I am missing something or if there is a quick workaround. Thank you.
Sub NewFileGenerate()
Dim I As Integer
Dim N As Integer
Dim X As Integer
Dim IndexedCell As String
X = 1
Windows(TextBox1.Value).Activate
'Need to count only populated cells
I = Sheets(1).Columns(2).Cells.SpecialCells(xlCellTypeConstants).Count
Set Newbook = Workbooks.Add
For N = 2 To I - 1
Application.CutCopyMode = True
Windows(TextBox1.Value).Activate
IndexedCell = Cells(N, 2).Value
Windows(TextBox2.Value).Activate
With Sheets(1)
With .Range("A1", "Z" & I - 1)
.AutoFilter Field:=5, Criteria1:=IndexedContract
.SpecialCells(xlCellTypeVisible).Copy
End With
End With
Newbook.Activate
ActiveSheet.Paste Destination:=Sheets(X).Range("A1:Z1")
Cells.Select
Selection.Font.Size = 10
Cells.EntireColumn.AutoFit
Cells.Select
X = X + 1
If X > 3 Then
Worksheets.Add(After:=Worksheets(Worksheets.Count)).Name = "Sheet" & X
End If
Application.CutCopyMode = False
Next N
End Sub

Answer 1

I'm guessing that your source worksheet (textbox2.value) has more rows than your index worksheet (textbox1.value). You set I equal to the number of rows in your index worksheet, then you tell the autofilter to only use that number of rows. You need to change the line "With .Range("A1", "Z" & I - 1)" so it picks up all of the rows in your source worksheet.