Oracle Forms

I have a multi-record block and I am trying to sort the data based on nbt item
I have a table called X which has x_type,x_code fields.
The table on which the block is created is Y. the table Y has x_type and y_desc as its fields

form layout is like below .the x_type is key field on which I have to query the records
and x_code is a non-base item in the form.I want to sort the records by X_code.

x_code y_desc
A xyz
c par
B lmn

my pre-query has this code

select x_type from y
where
x_type := x_type;

post-query has this code

select x_code from x
where x_type = :x_type;

It works fine in Enter-Queryand execute query mode. but when I am sorting the records on
nbt item x_code by SET-BLOCK-PROPERTY it doesn't do any thing