Oracle Forms

Hi friends
I want this script to run when every the user press the up and down arrow key in the details block, and I put this script in post-change trigger, and it is not working when the user press up and down arrow key what is the proper trigger to use for this situation
This my scrip :

BEGIN
SELECT NVL(QUANTITY_ON_HAND,0) INTO :PROD_GRINDER_HEADER.BALANCE_CHEMICAL FROM IM_LOCATION
WHERE STORE_CODE=:PROD_GRINDER_DETAILS.G_MACH_CODE
AND ITEM_CODE =:PROD_GRINDER_DETAILS.G_ITEM_CODE ;

exception
when no_data_found then
show_message(sqlerrm);
raise form_trigger_failure;
when form_trigger_failure then
show_message(sqlerrm);
raise form_trigger_failure;
when others then
show_message(sqlerrm);
raise form_trigger_failure;

END;


Waiting for your valuable answer.

Best regards

Jamil Alshaibani