repeatedly turned down for work

A few months ago I was turned down for a job with a government agency, after
what I thought was a very successful interview (in fact several stages of
interviewing). This was not the first time I had applied for
linguistics-related work with the US government: I am well qualified and
highly recommended. I would like to find out why I have never been
successful - perhaps there is something I could do to change my fortunes, or
stop wasting my time and energy. Is there any legal obligation for an
employer (especially a gov't employer) to disclose why an applicant was
turned down for a job?

Thanks