Update trigger does not work - Urgent

I had a problem as below:I had a trigger (Insert, update, delete) for my table. When I update data in correction mode (Update/Display All), the update trigger didn't work, only insert and delete could work. And there was a strange phenomenon, when I updated a row, there would be 2 rows in my audit table, one for Insert (new one) and one for delete (old one). The following is my trigger sql:
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create or replace
TRIGGER DC_TR_AU_DC_SECONDMT_DAT
AFTER INSERT OR UPDATE OR DELETE ON PS_DC_SECONDMT_DAT
FOR EACH ROW
DECLARE
V_AUDIT_OPRID VARCHAR2(64);