SQL & PL/SQL

Dears,
I m using 10g XE.

I am using merge into stmt, to inesrt non matched records and to update matched records with respect to primary key.
I have two uers in oracle. (TMP and REAL)
I have n number of tables with same definition in both users.
I will always merge all the tables into "REAL" from TMP uesr
Each and very table may contain more than 200000 records.

while merging some of the records got deleted from the REAL user which already available in REAL user.
i.e records available in TEMP.A is not available in REAL.A
if i re rerun the merge qry once again, the missing records inserted into REAL.A
how it happens? is ther is any bug with merge into stmt while processing huge amount of rows.

Sample Qry:
MERGE INTO REAL.med_stock_sales B
USING (
SELECT 1 AS RETAIL_OUTLET_ID,mss_id,MSS_ITEMCODE,MSS_TRAN_DATE,MSS_TRAN_TYPE,MSS_REF_NO,MSS_TRANS_QTY,MSS_FREE_QTY,MSS_PUR_RATE,MSS_SELLING,MSS_TAG,MSS_BNO,MSS_CAT1,MSS_CAT2,MSS_CAT3,MSS_CAT4,MSS_CAT5,MSS_CAT6,MSS_CAT7,MSS_CAT8,MSS_CAT9,MSS_CAT10,MSS_compid,MSS_diviid,MSS_locaid,TS,MSS_BAGS FROM TMP.med_stock_sales) E
ON (B.RETAIL_OUTLET_ID = E.RETAIL_OUTLET_ID AND B.MSS_ID = E.mss_id )
WHEN MATCHED THEN
UPDATE SET B.MSS_ITEMCODE = E.MSS_ITEMCODE , B.MSS_TRAN_DATE = E.MSS_TRAN_DATE , B.MSS_TRAN_TYPE = E.MSS_TRAN_TYPE , B.MSS_REF_NO = E.MSS_REF_NO , B.MSS_TRANS_QTY = E.MSS_TRANS_QTY , B.MSS_FREE_QTY = E.MSS_FREE_QTY , B.MSS_PUR_RATE = E.MSS_PUR_RATE , B.MSS_SELLING = E.MSS_SELLING , B.MSS_TAG = E.MSS_TAG , B.MSS_BNO = E.MSS_BNO , B.MSS_CAT1 = E.MSS_CAT1 , B.MSS_CAT2 = E.MSS_CAT2 , B.MSS_CAT3 = E.MSS_CAT3 , B.MSS_CAT4 = E.MSS_CAT4 , B.MSS_CAT5 = E.MSS_CAT5 , B.MSS_CAT6 = E.MSS_CAT6 , B.MSS_CAT7 = E.MSS_CAT7 , B.MSS_CAT8 = E.MSS_CAT8 , B.MSS_CAT9 = E.MSS_CAT9 , B.MSS_CAT10 = E.MSS_CAT10 , B.MSS_compid = E.MSS_compid , B.MSS_diviid = E.MSS_diviid , B.MSS_locaid = E.MSS_locaid , B.TS = E.TS , B.MSS_BAGS = E.MSS_BAGS
WHEN NOT MATCHED THEN
INSERT (B.RETAIL_OUTLET_ID , B.MSS_ID , B.MSS_ITEMCODE , B.MSS_TRAN_DATE , B.MSS_TRAN_TYPE , B.MSS_REF_NO , B.MSS_TRANS_QTY , B.MSS_FREE_QTY , B.MSS_PUR_RATE , B.MSS_SELLING , B.MSS_TAG , B.MSS_BNO , B.MSS_CAT1 , B.MSS_CAT2 , B.MSS_CAT3 , B.MSS_CAT4 , B.MSS_CAT5 , B.MSS_CAT6 , B.MSS_CAT7 , B.MSS_CAT8 , B.MSS_CAT9 , B.MSS_CAT10 , B.MSS_compid , B.MSS_diviid , B.MSS_locaid , B.TS , B.MSS_BAGS )
VALUES (E.RETAIL_OUTLET_ID , E.mss_id , E.MSS_ITEMCODE , E.MSS_TRAN_DATE , E.MSS_TRAN_TYPE , E.MSS_REF_NO , E.MSS_TRANS_QTY , E.MSS_FREE_QTY , E.MSS_PUR_RATE , E.MSS_SELLING , E.MSS_TAG , E.MSS_BNO , E.MSS_CAT1 , E.MSS_CAT2 , E.MSS_CAT3 , E.MSS_CAT4 , E.MSS_CAT5 , E.MSS_CAT6 , E.MSS_CAT7 , E.MSS_CAT8 , E.MSS_CAT9 , E.MSS_CAT10 , E.MSS_compid , E.MSS_diviid , E.MSS_locaid , E.TS , E.MSS_BAGS )