Memory upgrade

I have 2 HP Proliant miniserver and upgraded RAM from 1x4GB to 2x8GB (by YT user).

First server: 

BIOS sees 16GB, dmesg sees 16GB spdmem0 and 1 but only 4GB of RAM

# dmesg |grep mem
real mem  = 3622907904 (3455MB)
avail mem = 3539423232 (3375MB)
spdmem0 at iic0 addr 0x50: 8GB DDR3 SDRAM PC3-12800 with thermal sensor
spdmem1 at iic0 addr 0x51: 8GB DDR3 SDRAM PC3-12800 with thermal sensor

I have upgraded from 7.4 i386 to 7.5 amd64 due to the fact that server has AMD Turion processor.

This made a job.

# dmesg |grep mem
real mem = 17028415488 (16239MB)
avail mem = 16491167744 (15727MB)
spdmem0 at iic0 addr 0x50: 8GB DDR3 SDRAM PC3-12800
spdmem1 at iic0 addr 0x51: 8GB DDR3 SDRAM PC3-12800

In a second server I have upgraded from 7.5 i386 to 7.6 i386 but server sees only 4GB of RAM

# dmesg |grep mem
real mem  = 3622907904 (3455MB)
avail mem = 3539423232 (3375MB)
spdmem0 at iic0 addr 0x50: 8GB DDR3 SDRAM PC3-12800 with thermal sensor
spdmem1 at iic0 addr 0x51: 8GB DDR3 SDRAM PC3-12800 with thermal sensor

Is anybody with similar experiences? Any ideas how to fix RAM?

Regards

Zbigniew Kossowski