A salary component may not be visible in pay slips due to following reasons:
1) It is either inactive or has not been marked as paid component
2) It is not allocated in the salary structure of employees whose pay slips are being viewed
1) If the salary component is inactive or has not been marked as a paid component
a. Go to Settings > Salary Components and double click on the salary component that is not visible in pay slip.
b. Check the boxes displayed before Paid Component and Active and click on Save button at the bottom
2) If the pay component is not added in the salary structure of employees whose pay slips are being viewed