Exchange backup completing partially

Dear Friends,

I have one query , we  have  exchnage backup configured in our enviorment , it is runing fine but whenever Exchange admin do the chnages like mail box movement from one database to other database  in any of the databases which part of backup , ( he make this changes in day time and backup schedule starts in night )  exchnage backup completes partially after that i rerun it, it sucess . i want to know the reson that why does it complete partailly in first attempt .

Please Assist .

Thank You !!