LastRow Does Not Seem To Look In The Workbook That The Macro Has Open

The problem is that LastRow does not seem to look in the workbook that the macro has open, and always returns a value of 1. If I place the sheet that i want to run the macro on, in the workbook that contains the macro there is no problem. What do I need to do to fix this.

LoopCount = Sheets("Dates").Range("A" & Rows.Count).End(xlUp).Row
rng = 2
i = 1
Do
Selection.AutoFilter Field:=7, Criteria1:="=" & Sheets("Dates").Range("A" & rng), Operator:=xlAnd
z = Range("B" & Rows.Count).End(xlUp).Row
For y = 2 To z
If Rows(y).Hidden = False Then Exit For
Next y
MsgBox y
MsgBox z
Range("H" & z).Formula = "=Countif($A$" & y & ":$A$" & z & ",A" & z & ")"
Range("H" & z).Copy Destination:=Range("H" & y & ":H" & z)
rng = rng + 1
i = i + 1
Loop Until i = LoopCount