Looping through cells in another workbook to copy specific values - vba

Question

I've been fighting with this for a couple days now, and am at a loss on what else to try. My goal is to have a prompt for where a workbook is saved, this spreadsheet is obtained from an external source and name/location can vary. After opening the workbook, switch over to the second sheet and start searching for the values to copy to the workbook the macro is run out of.
The code I have works great if I set a breakpoint at the calculation for the last row, and at the For loop. Without those 2 breakpoints, it appears that none of the information in the workbook loads before running the rest of the code.
Public Sub Clm2Count()
Dim i, j, k, last As Long
Dim wkbSource, wkbCrnt As Workbook
Dim str As Variant
Dim strArray()
strArray() = Array("THIS", "IS", "MY", "ARRAY")
Set wkbCrnt = ThisWorkbook
k = 1
With Application.FileDialog(msoFileDialogOpen)
.Filters.Clear
.Filters.Add "Excel 2007-13", "*.xlsx; *.xlsm; *.xlsa"
.AllowMultiSelect = False
.Show
If .SelectedItems.Count > 0 Then
Application.ScreenUpdating = False
Set wkbSource = Workbooks.Open(.SelectedItems(1))
Application.ScreenUpdating = True
End If
End With
Sheets(2).Activate
Cells(5,1).Select 'Trying to activate a cell before calculating last, didn't work
last = wkbSource.Sheets(2).Cells(wkbSource.Sheets(2).Cells.Rows.Count, 1).End(xlUp).Row
For i = 51 To last
If InStr(1, wkbSource.Sheets(2).Cells(i, 2).Value, "TEST") > 0 Then
For Each str In strArray
If InStr(1, wkbSource.Sheets(2).Cells(i, 2).Text, str, vbTextCompare) > 0 Then
If InStr(1, wkbSource.Sheets(2).Cells(i, 2).Text, "A", vbTextCompare) > 0 Or InStr(1, Cells(i, 2).Text, "B", vbTextCompare) > 0 Then
If str = "MY" Then 'Specific value from the array
wkbSource.Sheets(2).Cells(i, 3).Copy
wkbCrnt.Sheets(1).Cells(k, 1).PasteSpecial
wkbCrnt.Sheets(1).Cells(k, 2).Value = "QC"
wkbCrnt.Sheets(1).Cells(k, 3).Value = i & ", " & str
k = k + 1
Exit For
End If
ElseIf InStr(1, wkbSource.Sheets(2).Cells(i, 2).Text, "C", vbTextCompare) > 0 Then
wkbSource.Sheets(2).Cells(i, 3).Copy
wkbCrnt.Sheets(1).Cells(k, 1).PasteSpecial
wkbCrnt.Sheets(1).Cells(k, 2).Value = "QC"
wkbCrnt.Sheets(1).Cells(k, 3).Value = i & ", " & str
k = k + 1
Exit For
Else
Exit For
End If
End If
Next str
wkbSource.Activate
End If
Next i
End Sub
Any ideas on what I might be overlooking?
Edit:
Here are images of the beginning and end of column A, with the identifiers removed
Beginning
End

Answer 1

Instead of Sheets(2).Activate use wkbSource.Sheets(2).Activate. Same goes for cells and any other kind of ranges you are using.
It is especially crucial to be as explicit as possible which is the target workbook when you have a multiple workbook interaction.
To find last row use this line:
last = wkbSource.Sheets(2).Cells(wkbSource.Sheets(2).Cells.Rows.Count, 1).End(xlUp).Row
Edit: The issue was caused by a hidden sheet - it is better to use sheets name in this case.