Symantec Differential/mod time backs up full data every time

Hey,

We are currently using Netgear ReadyNas 2100 are facing problems with the backup.

The differential backsup full data every time it runs.We generally have full backups(using mod time for linux system)during the weekends and differential backup in the weekdays.We aslo use monthly backups at the end of the month.

The full backups are completed succesfully without any issue.

Is the issue related to netgear?

Would it good option to try incremental backup?

Can we make a differential backup link to a particular full backup,

it  compares the files with the full backup date specified by me?