date picker not working after submit of php form

date picker not working after submit of php form

I'm using php and have a form with two date field using jquery datepicker function.  The date drop down works fine when the page is first rendered, but after the submit, the drop down no longer works.  You can enter a date into the field and resubmit the form, but why doesn't the drop down calendar appear after the submit? I'm using the standard function:
$(function() {
            $( "#txtStartDate" ).datepicker();
            $( "#txtEndDate" ).datepicker();
        });

Thanks,
Tom