Backup Object summary order

Doing my weekend backups, i have set the backup object order to start the largest systems first.  For the most part i can see in the log that it is behaving, but there are some that although have been indicated to start near the beginning, dont' start until about 24 hours later.  I have treewalking disabled on the systems in question.

is there something else that could be delaying the startup?

thanks