Slowness in VM backup

Hi,

i'm facing a slowness in VM backup,mentioned that in past the full vm backup finished for (560 Gb) finished within 2 hours,but now the backup take more than 18 hours.

i'm using (nbd) transport type,backup taking to symantec appliance,i tried to add another vm machine in same host under the vcenter and backup finished fast

Need your advise.

Vshpere version is : 5.5

Netbackup version : 7.6.0.3

Applicance version : 2.6.3