There are questions raised why MS SQL backups are running as full backups and there are no differentials.

 

The reasons may be following:

• SQL Server Recovery Model is Simple.
  To have differential backups, one should switch Recovery Model to Full.
  For more information, check the Microsoft KBA: https://msdn.microsoft.com/en-us/library/ms189275.aspx; 
  
• another backup software is running MS SQL backups that truncate logs;