alexandrepolar

Encontrei a solução.
 
No meu caso era a opção "Clear archive attribute" na aba Advanced da tarefa, estava desmarcado.
 
Tópico pode ser fechado.
 
 
http://forum.cobiansoft.com/forum/yaf_postst9_Incremental-or-differential-Always-full--What-to-check.aspx
 
If the backup is changing to FULL (ie, when you see the history, the last item is FULL) you will find a reason in the log file. You may fins a line like this:

- Changing to FULL. Reason: ****** , where ***** is the explanation.

Some reasons why the task is changing to full are:

* Your source is FTP. FTP sources can only be backed up fully.

* This is the first backup on the history list: must be full

* You are compressing and overwriting using 7z. 7z doesn't support refreshing in this version

* You are compressing, overwriting and splitting. Archives that use "splitting" cannot be refreshed

Now, if the last history item is Incremental or Differential but ALL files are being copied:

* Check that the "Clear archive attribute" is checked in the Advanced tab of the task. That thing is under "Advanced" for a reason. Don't touch it!!!!

* Check that the account running the backup (the account under which the service is installed or the account impersonating the task) has permissions to change the source. The archive attribute must be cleared, so you need permissions to change the source. No, being an administrator doesn't automatically grant you full permissions. If this is the case, temporarily changing the log to Verbose will show you the line "Couldn't change the archive attribute)

* Differential backups don't clear the archive attribute, so you need at least one full backup to reset the backup.