Workflow email alert problem

For example if the employee submit vacation request to his manager through ebs.

When he submit a request from ebs the manager receive the email from the ebs then after the manager approve,  the employee receive the email that his manager approved his request

but in ebs the status its not reflecting what happened in the emails (cause it shows that the email is pending in the manager side).

It looks like Oracle ebs unable to read the acknowledgment!

Any solution?