[symantec Netbackup] All Tape drives are not used effectively for Backups

We are running Netbackup version 7.6.2.

We have four drives and all of them are active. But when I just looked at the activity monitor, I found that only 2 of the 4 drives are actually used at any point in time. There are jobs waiting in the queue with a message ‘No Drives available’ even though they are two drives un-used.

I can see the Max Concurrent Drives is configured to be 4 in both Master and media servers.

 ./vmoprcmd -d ds

                                  DRIVE STATUS

Drv Type   Control  User      Label  RecMID  ExtMID  Ready   Wr.Enbl.  ReqId
  0 hcart    TLD                -                                         No       -         0  
  1 hcart    TLD                -                                         No       -         0  
  2 hcart    TLD                    Yes   H01522  H01522     Yes     Yes        0  
  3 hcart    TLD                    Yes   G00006  G00006     Yes     Yes        0  
 

Even though the drives are shown as not ready, they are being used but the problem is only two of them are used concurrently.

Due to this the backups are lagging behind... 

I am relatively new to Netbackups. Any pointers on what could be the issue and how can I check it up further?