There could be several reasons. I would recommend checking out the troubleshooting documentation link below.
You can also check the backup history of a subclient or Client & that will tell you if you had failed files and you can find out exactly why those files failed to backup. If you wish to view the backup history for all the sub clients simply expand to the iDA level (i.e. File System). Right click on the sub client then click on 'View > Backup History > OK'. -Now you can right click on any job and then click on 'View Failed Items' if the job had any failed items