If you post an event in a response window that closes the
response window, and call posted events in the Open event for a
main window that displays when the response window is closed, the
posted events in the Open event are not executed.

This is due to
a limitation of the threading model in Web Forms applications.

To make sure that the posted events of the main window are
executed, close the response window directly in a triggered event
rather than in a posted event. Alternatively, move the code from
posted events in the main window to events that are triggered directly
by the user.