ssh authentication

Following Bitbuckets authentication changes I have amended our TeamCity projects to use ssh when accessing our git repositories. This is working except for one instance which has me puzzled.

We have a deployment project which runs fine when run from the web front end but, when run at a specific time using a schedule trigger it fails with the following error:

Failed to collect changes, error: org.eclipse.jgit.errors.TransportException: xxxx@bitbucket.org:xxxxx/xxxx.git: SSH_MSG_DISCONNECT: 2 Connection blocked because server only allows public key authentication. Please contact your network administrator. , VCS root: "Git_Test_Develop" {instance id=333, parent internal id=48, parent id=TestEnvironment_GitTestDevelop, description: "xxxx@bitbucket.org:xxxxx/xxxx.git#refs/heads/develop"}

I can't figure out why this only happens with the trigger. I would have thought it would fail whatever means was used to start the job if the ssh setup was incorrect which the message seems to suggest. The deployment job is run on a build agent which is a different machine to the one hosting the web site, I don't know if that is the reason.