Job Cancelling Due to Exceeding Run Time

We are running Backup Exec 2012 on a Windows Server 2008 R2 VM. It has been running fine for the the best part of the last year ,however over the past few months it seems to be taking longer to backup, but the extra amount of data is only ab increase of about 10GB or so. On 28th December it took 15:43:18 to backup 680GB @ 893.23MB/Min. 26th January it backed up 686GB @ 882.76MB/Min , but cancelled it self due to reaching the maximum configured run time.

We could turn off the maximum run time but this would mean it clashing with the start time of the next job due it it being a full backup, so thats not really the answer to it.

So what causes it, as 6GB of data shouldnt really take an extra 6+ hours to backup and verify.

Any ideas?