Excel Function works on one sheet but not another

I have written a short function in Excel to find a number in a range in column AZ and return the row number.  If the number is not found in then finds the next number following.  So if the list contains 070, 071, 073 then looking for 070 would find the row for 070 whereas looking for 072 would return the row for 073.  There is a similar function to work in reverse, so if the list contains 070, 071, 073 then looking for 073 would find the row for 073 whereas looking for 072 would return the row for 071.  The function works fine on the worksheet where I created it, but will not return a value when called from a different worksheet.  The function code is:

Function Start_Range(StartNum As String, CC As String)
    Dim Search_String As String
    Dim SearchNum As Integer
   
    SearchNum = Val(StartNum)
    Search_String = Format(CStr(SearchNum), "000") + CC
    Do While IsNumeric(Application.Match(Search_String, Worksheets("RDB1CY").Range("AZ:AZ"), 0)) = False
        SearchNum = SearchNum + 1
        Search_String = Format(CStr(SearchNum), "000") + CC
    Loop
    Start_Range = Application.Match(Search_String, Range("AZ:AZ"), 0)
End Function

When called from Worksheet RDB1CY the function operates as expected.  When it is called from any other worksheets it returns #N/A.

Can anybody point me to where I am going wrong?