Employees not calculating headings

Dear Team,

I imported the staff list of about 2800 employees, both for employee and contracts.

But then i notice that upon running Automatic generation, it flagged an error for 1877. All this employee's payroll were not calculated. 

I then compared their layout with the layout of the others that calculated in case i missed a step. But all were the same.

Kindly assist in what could be wrong with these guys and what can be done accordingly.

Thanks.

Adeola.