Can you be more specific. At what instance you are expecting the POST-QUERY to be fired? It will fire only if you issue EXECUTE_QUERY.
http://www.club-oracle.com/forums/post_query-problem-in-oracle-forms-t9751/#post23794 , This is the link at where I have tried to show my problem with the help of an image,Please get the link:
Your code that you put in the POST-QUERY trigger won't fire untill you fired EXECUTE_QUERY on that data block
So you should make a data block as a result for search for example and then fire EXECUTE_QUERY and your code then will work
Where you have placed your post-query trigger. Your trigger should be on block level and on same data block where you want to load data from other than source table.