Dell Power Edge R 710 - Memory not Available

Hello,

Dell Power Edge R 710, Windows Server 2008 r2 x64, memory was upgraded from 32Gb (4*8Gb) to 64 Gb (8*8Gb)

BIOS shows all 64Gb, But system shows: 64Gb (32Gb available) 

Dell OpenManage Server administrator also shows: Total Installed Capacity: 65536 MB, Memory Available to the OS: 32768 MB

Why all memory not available to the system?

Please suggest how to solve the problem,

Thank you!