SQL & PL/SQL

Hello,

I'm trying to read from a clob using dbms_lob in my trigger and assigning the value to a variable. My trigger is compiling fine, but it doesn't seem to be working.
I'm using 10gR2. Am I misunderstanding how to read from/assign to a variable in a trigger?

Line from trigger code:
...
v_serial_start := dbms_lob.instr(:new.search_xml_clob,'serial=') + 8;
...

Thanks,
Mimi