SQL Backup generates a log file for each process that it runs. The default folder where the logs are stored is C:\Documents and Settings\All Users\Application Data\Red Gate\SQL Backup\Log\<instance name> on Windows 2003 and older, and C:\ProgramData\Red Gate\SQL Backup\Log\<instance name> on Windows Vista and newer.
Could you please post the contents of the log for the backups that failed?
Could you please post the contents of the log for the backups that failed?