First of all, make a list of the work you do at the office on a daily basis, along with your achievements and skills that you are utilizing for the growth of the organization. After that, try to compare if the line staff has the same qualifications and accomplishments as you. This comparison will help you understand why you might be paid less or provide you with strong points for negotiation. Present these reasons to the HR department, as they will offer you a reasonable answer or solution.
Regards,
Kirti