Netbackup 7.5 only 1 tape drive out of 2 is active at a time

Hi all, 

Kindly I have a problem  in which I have two tape drives configured  but only 1 is active at a time

the configration is as following:

1 Master server 7.5

2 Media server   6.5.5

with the SSO configured 

I checked the maximum concurrent jobs from the storage unit properties and it's 2 

I have enabled multiplexing and its value is 2 on all the policies 

so what else can I check and why is only one tape drive is active at a time and when I run another job it gives status queued showing the message no drives are available 

so can any one give me a hint what else I can do ?

Thanks