There may be many reasons...it is not possible predict based on you have provided.
Make sure you add the job output file to the job step and see what is causing if you can't understand post the output here, someone will take look and give the possible reasons.
Reasons could be: Network issue, blocking, server not available, db maintenance etc...
Good luck...
MohammedU.
Microsoft SQL Server MVP
Moderator
SQL-Server-Performance.com
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