Mview's taking a looong time to refresh. Any ideas?

Question

I have the following mview below. We are running the refresh using the FORCE option nightly. There are times when the refresh takes hours and we have to kill the job. Then, we just do a drop/recreate of the mview and it completes in < 20 minutes. Anyone have any ideas as to why?
The merchant table has about 800k rows.
The merchant_transaction table has about 18 million rows.
I have included the explain plan of this query below.
CREATE MATERIALIZED VIEW CARS.MER_MERTXN_MV
TABLESPACE MVIEW_TB
NOCACHE
LOGGING
NOCOMPRESS
NOPARALLEL
BUILD IMMEDIATE
REFRESH FORCE
NEXT NULL
WITH PRIMARY KEY
AS
SELECT mt.merchant_num,
mt.transaction_ref,
mt.transaction_type,
mt.transaction_status_code,
mt.transaction_date,
mt.sale_date,
NVL(mt.transaction_credit_amt,0),
NVL(mt.transaction_debit_amt,0),
NVL(mt.open_credit_amt,0),
NVL(mt.open_debit_amt,0),
NVL(mt.reserve_bal,0),
NVL(mt.collection_bal,0),
NVL(mt.acct_bal,0),
m.coll_assigned,
m.acct_location_code,
m.acct_type_code,
m.marker_bank_code,
m.cost_center_code,
m.portfolio_code,
m.rowid,
mt.rowid
FROM merchant_transactions mt, merchant m
WHERE mt.merchant_num = m.merchant_num;
EXPLAIN PLAN
Operation     Object Name     Rows     Bytes     Cost     Object Node     In/Out     PStart     PStop
SELECT STATEMENT Optimizer Mode=ALL_ROWS          18 M          212259                     
HASH JOIN          18 M     2G     212259                     
TABLE ACCESS FULL     CARS.MERCHANT     791 K     40 M     9206                     
TABLE ACCESS FULL     CARS.MERCHANT_TRANSACTIONS     18 M     1G     96231

Answer 1

1. Yes, but they aren't being used as per explain plan. I added a hint which helped. The problem is when we do a complete refresh. The fast refresh works fine. However, we have times when there is so much data added, it decides to do a complete. We are running FORCE. Does it DELETE the data or TRUNCATE the data when it does a complete? It seems its doing a DELETE.
2. no rows
3. 10.1. No parallelism being used.