One of my apps impliments log shipping. It does not use any of the log shipping stuff in SQL Server, so every 15 minutes it issues a BACKUP LOG statement. It all works

Every so often, maybe once every two days, I get a zero length log. No error. When the next log is backed up it ships across fine. Any idea what causes it? The shipped logs are normally anywhere from 5MB to 300MB and there is always constant activity on the database.