Hi Praveen,
What version of TestStand are you using? Can you tell me more about your custom step-type?
Please verify that you have done all of the following:
1. Edit the post-step VI. Go to File » VI Properties and select Window Appearance from the drop-down list. Select the Customize... button and make sure that "Show front panel when called" is unchecked. Save and close VI.
2. In the post-step, make sure that "Show VI Front Panel When Called".
3. On the bottom of the Substeps tab, check "Apply Changes in the Dialog to all Loaded Steps of this Type."
I tested this on my machine with a custom-steptype derived from an Action step in TestStand 4.1, (and did all of the above steps) and it worked as expected.
Jessica
National Instruments
Product Support Engineer