Manage to solve the problem from a working colleague. Problem was resolved by adding a new scsi controller to the vm. After adding the controller either shutdown or restart the vm. Doing so I was able to discover the LUN and install the RDM disk to the servers.    
Hello. I have this weird problem on Equallogic PS6100 storage. Here hoping that I can have some sort of answer or advice to further troubleshoot and hopefully a solution to the problem. Objective I require RDM disks to be install to virtual servers through Equallogic PS6100. The RDM disks will be used for Windows clustering.   Below are the infrastructures on my environment. 1 unit of Equallogic PS6100.  3 units of ESXi 5.5 servers. (2 NICs connected to the PS6100) 2 virtual machines running Windows Server 2012 R2 meant for windows clustering.  I have created 6 volumes on the Equallogic PS6100 but only manage to connect 5 volumes to the two windows servers. I need all 6 volumes to be connected to both servers as RDM disks. Somehow the 6th volume is giving me a problem whereby both servers manage to detect the LUN but I can't install them. By the way the connections are on ISCSi. Once I install the 6th volume on to one of the window server the other server will no longer be able to detect the LUN anymore. Hence I can't install this volume on to both servers. I have tried scanning the LUN volumes on all 3 ESXi servers numerous times and still I can't install this 6th volume to both servers. I have even migrated the windows servers to all 3 ESXi servers and still having the same result. Any advice in here will be highly appreciated. Thank you.     
Connecting RDM volumes on Equallogic PS6100 to virtual machines.