Get Multiple MAX With Where Criteria

use of Row_Number() over ( partition... but I dont understand how.

Imagine I have a table like
CustomerID, PartNum, QtyinOrder, shipped
1 6 3 0
1 6 2 0
2 6 1 0
2 5 1 0
2 5 2 0
2 5 3 0
2 5 4 1
1 6 4 1
2 6 2 1

But I wanted to return

CustomerID, PartNum, MaxQtyOrderedNotShipped

That would be just the rows
1 6 3 0
2 6 1 0
2 5 3 0

If I use this:

Select CustomerId,PartNum, shipped, QtyInOrder AS MaxOrderedNotShipped
from
(SELECT [CustomerID]
,[PartNum]
,[QtyInOrder]
,shipped
, row_number() over (partition by [CustomerID], PartNum order by QtyInOrder desc) as recid from [SILK].[dbo].[MaxofGroup]) as f where recid =1

there is no restriction, so I get the shipped...If I alter the where clause to work only on not shipped, I get no records...as below

Select CustomerId,PartNum, shipped, QtyInOrder AS MaxOrderedNotShipped
from
(SELECT [CustomerID]
,[PartNum]
,[QtyInOrder]
,shipped
, row_number() over (partition by [CustomerID], PartNum order by QtyInOrder desc) as recid from [SILK].[dbo].[MaxofGroup]) as f where recid =1 and shipped=0