Hi,
if you are logged on as a user1 and switch to another user2 with sudo the ssh agent from user1 cannot be used because the “switched” user2 does nothing know about the ssh-agent respectively the agent socket.
The path to the agent socket is stored in the SSH_AUTH_SOCK environment variable. So you have to keep/forward this variable to the switched user. This can be done by adding
Defaults env_keep+=SSH_AUTH_SOCK
to /etc/sudoers
Michael