let me try this again
;
WITH newtable
AS ( SELECT StoreProdID
, sp.ListID
, ProdID
, Qty
, PrevQty
, GroupID
, StoreID
FROM #StoreProducts sp
JOIN #StoreList sl
ON sl.ListID = sp.ListID
)
SELECT tw.ListID
, tw.ProdID
, tw.Qty
, pw.qty AS PrevQty
FROM newtable tw
JOIN newtable pw
ON pw.ProdID = tw.ProdID
AND pw.StoreID = tw.StoreID
AND tw.GroupID - 1 = pw.GroupID