EC2 Instance IP keeps changing despite elastic IP

I am working with a developer on a project to be created for me. The project requires connection to the IP of the project which is on my AWS EC2 account. The IP keeps changing and when it does, the instance reachability check fails. He set up an elastic IP, which seems to be in place but the IP continues to change after the elastic IP and the instance reachability check continues to fail so the project cannot be completed. HELP!