What would prevent a time card from being able to be Submitted (button is grayed out/inactive)?

Summary:

We have a subset of employees who are consistently unable to submit their time cards. The Submit button is grayed out or unclickable. It behaves the same way in the browser or in the phone app. They are then dependent on their manager or the T&L manager to submit the time card on their behalf.

The manager is not making any changes to the time card data that the employee had entered. Is there something in particular I need to look for that would allow an employee to enter their time but not be able to submit it?