An EERP is an acknowledgment from your trading partner to say that their system has successfully received the file. The reason for the delay in receiving the EERP is that it is not ready to be sent to you at the end of the session in which you send the file.
The most common cause of this is if you are sending files to a partner via a Value Added Network (VAN). Although you have sent the file to the VAN and the VAN has forwarded it to the Trading Partner, they still need to connect to the VAN to collect the data.
They will generate the EERP when they connect and pick up the file, and you will receive the EERP the next time you communicate with the trading partner.