There could be several reasons:

• Did you fill out your W-4 and I-9 forms with the Office of Human Resources?
  Federal regulations require that these forms be completed before you can work and be paid.
• Has the Office of Human Resources received your Student Hire Form?
  When you are hired, your supervisor must submit a Student Hire Form to Human Resources, you will not be paid until we have not received that form.
• Was your time sheet turned in on time? Time sheets received by the Office of Human Resources after the payroll deadline will not be processed in the regular payroll.