Hello Santosh,
We don't use DLO for PST, so only files.
There's no link between backlog and Success/failled backup. A backup is just the process to save the data to the local storage (I guess). I haven't been able to find a way to estimate the backlog or at least to know if we have some.
I've opened a case @support, it looks like there's nothing. I've heard that a new releas should appear soon, but I have no other info.
To sum up, most of our clients have a backup status to success, but on the server lots of files are missing.
Regards,
NH