Probably not enough information to give any definitive answer but I'll try!
Some further information would be useful:
Which version of ESX?
Which Nimble OS Version?
Are you using the Nimble PSP on your ESX hosts (Nimble Connection Manager for VMWare)?
IS the Nimble plugin deployed?
iSCSI or FC?
First place to look is ACL's, it is likely to be a security issue with your Initiator Groups. Do each of the 5 volumes belong to the same initiator group?
If iSCSI and an ESX Cluster make sure the multi-initiator checkbox is ticked for each Volumes propoerties.
Rescan in vCenter and see if this fixes it.
Are your IQN's all correct? One obscure thing I have seen in the past with ESX 5.x was the iSCSI IQN is actually case sensitive so if entered in the wrong case into the Initiator Group it is not recognized.
To test this, you could give Unrestricted Access (Temporarily!) to the 5th Volume and see if it is even visible, if it is you know if is an ACL issue.
Come back to me if not or else give support a call.