Re: VMWARE Vcenter
are you using cascading initiator groups?
Especially for clustered environments it's possible to use cascading initiator groups, and the proper flagsettings need to be applied on any initiator or initiator group accessing the clustered devices.
Another possible reason for this behavior might be that these devices are presented as pRDM's to a virtual W2K8 (or later) MSCS node.
These devices are protected by SCSI 3 persistent reservations which could prevent proper discovery from an ESX host.