Oracle 11g ON-EVENT (CREATE TABLE) trigger sometimes does not fire

I have the following trigger on the DSMITH schema. 

I have noticed on numerous occasions that when another user (ie. JDOE) creates a table for DSMITH, the trigger does not fire (the grants to the new table are not being given to the appropriate roles).

What would cause the grants to not have been created?  Is there a better way to do this? 

We have numerous roles that need to be given their appropriate grants when new tables are created.

Thanks!

CREATE OR REPLACE TRIGGER "DSMITH".CREATE_DSMITH_GRANTS AFTER
CREATE ON "DSMITH".SCHEMA DECLARE
l_str1 VARCHAR2(255);
l_str2