How to refresh mirrored repository from GitHub?

I’ve setup a mirror repo from GitHub on GitLab.com. For a few commits, it successfully mirrored from GH to GL. But it has now stopped. When I visit my project on GitLab, I see a message ‘The repository failed to update xxx minutes ago.’

The message give is:

Could not update branch master. Please refresh and try again.

How can I resolve this?

Thanks