The optimizer could not merge the view at line ID 2 of the execution plan

I am running the sql tuning advisor on the following query and getting the error.

The optimizer could not merge the view at line ID 2 of the execution plan

Please help me to tune this query.

SELECT "A1"."LOC",ROWNUM
FROM (
(SELECT DISTINCT "A8"."LOC" "LOC"
FROM "ITEM_LOC_SOH" "A8","STORE" "A7","ITEM_MASTER" "A6"
WHERE "A6"."ITEM" LIKE :1 AND "A6"."ITEM_PARENT"="A8"."ITEM" AND "A8"."LOC"=TO_NUMBER(:2) AND "A8"."LOC"="A7"."STORE" AND "A7"."STORE"=TO_NUMBER(:3)) UNION ALL
(SELECT DISTINCT "A5"."LOC" "LOC"
FROM "ITEM_LOC_SOH" "A5","STORE" "A4","ITEM_MASTER" "A3"
WHERE "A4"