Why would there be no federal withholding's on my wife's W2 even though she asked for the maximum?

I just don’t know why there was no taxes withheld? The thing is she is in school and this job is barely even part-time, she literally does this job for a little extra spending money. She made less than 5k last year, but as far as I know there still should have been taxes withheld. Can anyone shed some light on this for me?