Formula returning Incorrect data

Hi all I am trying to do a Lookup Formula in Excel 2013.
This is the formula I am trying to use:
"=LOOKUP("'Tab 4_ Ref Level 1 Data'!",'Tab 4_ Ref Level 1 Data'!AN:AN,'Tab 4_ Ref Level 1 Data'!AN8642)"

This is the data it is supposed to be pulling from:
Image : Excel_Data-For-Lookup.JPG

But for some reason it keeps returning a "0" for the result.
I can change the AN:AN to AN1 and it returns the correct result, but when I copy the formula down to other rows it changes the AN1 to AN2, AN3, AN4,.... and causes the formula to return a N/A result.

Any ideas why its doing this and how I can fix it??
The Tabs/ Sheets are all in the same Excel Workbook.

Thanks in advance

IKE