SQLite. Stops at the first instance the WHERE condition is met

I have tried this code and it works but only on the first instance for each customer. i.e the column should run through and add all the orders to the previous amount owing.

UPDATE customers_tbl 
    SET customer_amountowing = customer_amountowing + 
       (SELECT sorder_amount FROM standingorder_tbl 
           WHERE standingorder_tbl.sorder_customer = customers_tbl.customer_address1) 
    WHERE EXISTS 
        ( SELECT * FROM standingorder_tbl
           WHERE standingorder_tbl.sorder_customer = customers_tbl.customer_address1 );

Can anyone tell me whaere I am going wrong here. Cheers guys!!