Excel Error 1004 in vba vlookup function

[TonyRandom]

I hope I haven't posted this in the wrong section, but it is a VBA problem, so please forgive me if it should be elsewhere.
I have two tabs in a spreadsheet, both containing lists of parts codes. I want to programatically compare the first list against the second, and where the code isn't found in the second list I want to append it to the end of the list.
I have tried to trap the error 1004 that is raised when the part cannot be found in the list, and have created an on error goto jump to handle it, and this works on the first time it encounters the problem, but as soon as it hits it again, the error appears and the code stops.

Here is the section of code i have written so far.

    Sheets("Main").Select
    Range("A1").Select
    Range(Selection, Selection.End(xlDown)).Select
    Selection.Name = "AllParts"
    Sheets("ImportList").Select
    Range("A1").Select
    On Error GoTo AddPart
    FRow = Cells(Application.Rows.Count, 1).End(xlUp).Row
AddPart:
    If Err.Number = 1004 Then
    myPartCode = ActiveCell.Value
    With Main
    With .Cells(.Rows.Count, 1).End(xlUp)(2, 1)
        .Value = myPartCode
    End With
    End With
    End If
    
    ActiveCell.Offset(1, 0).Select
    myPartCode = ""
    On Error GoTo AddPart

    For i = ActiveCell.Row To FRow
    myPartCode = Application.WorksheetFunction.VLookup(ActiveCell, Range("AllParts"), 1, False)
    
    Debug.Print myPartCode
    ActiveCell.Offset(1, 0).Select
    Next i

The sheet named Main also has its codename set to Main.

I cannot understand why the on error captures it once, then doesn't capture it the second time.
This is completely baffling me, ane left me pulling out my hair.

[NeoPa]

Error code is supposed to reset the error. One of the Resume statements should be used. Using On Error to handle program flow is only an acceptable idea when you have a very good understanding of the Error handling facilities in VBA.

I would recommend Error Code that does what's expected and manage the flow of the logic entirely separately.