[AccessD] Continue Incrementing

When I open my form and add a new record and (say there is already a record
in the table "BOM-2601-0010"), the next number given is correct
("BOM-2601-0011").
 
However, if I add another record and use the same prefixes as before
"BOM-2601"......the next number given is the same as the previous
("BOM-2601-0011"). For some reason, I can only get it to increment one time
only and that's all.
 
Is there something I can insert in the following statement (which is a
doubleclick event that assigns the Document Number), which tells it to
continue incrementing even though the prefixes are the same?
 
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Private Sub DocumentNumber_DblClick(Cancel As Integer)
Dim intMax As Integer, strPrefix As String
 
If Me.DocumentNumber = "" Or IsNull(Me.DocumentNumber) Then
 
strPrefix = Me.[cboDocType].Column(0) & "-" & Me.cboWBSCodesSelect & "-"
 
If DCount("DocumentNumber", "DocumentList", "Mid([DocumentNumber],6,4)='" &
Me.cboWBSCodesSelect & "'") > 0 Then
intMax = DMax("Val(Mid([DocumentNumber],11))", "DocumentList",
"Mid([DocumentNumber],6,4)='" & Me.cboWBSCodesSelect & "'")
 
Me.DocumentNumber = strPrefix & Format(intMax + 1, "0000")
Else
Me.DocumentNumber = strPrefix & "0001"
End If
End If
End Sub
 
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