If you're unable to unapprove a time or evaluation submission, that's because the prior approvals triggered a wage increase.

You see, when a wage increase is going to be triggered, a message displays in the approval window that says, "Are you sure you want to approve as this will trigger a wage increase?". By submitting the approval, the wage increase triggers and there's no going back.

So if you find yourself unable to unapprove, that's intentional so that you don't reverse the wage increase that was triggered.