HDDs not recognized

PowerEdge R720xd.  I am having trouble getting this server to recognize the front twelve HDD slots.  I have three installed and they are seated properly, but it will not recognize them at all in the PERC controller.  I have checked that they are compatible and I have tried different slots.  I have used an OEM PERC controller and one flashed to IT mode and they both recognize the rear two HDDs but not the front twelve.  Any ideas?  I’m just barely above a novice at this so any help is appreciated.