Normally, when a locked working copy state is detected (e.g. because the previous build was interrupted during checkout), subversion scm will remove the working copy and does a fresh check out.
This does not work when the lock is in a working copy (svn:external) sub directory entry. The update of the external will simply be skipped and the working copy will stay in locked state.
I just realised that I recently commented JENKINS-7009 which might be the same/similar issue.