Im confused

hi peeps iv just bought a matched pair a sticks for my rig (2x4gb) to add to the (2x4gb) i already have takin me to 16gb, the sticks are exact make and specs as what i already use, but they are low profile sticks i put them in my board reconises them knows they are they but wont use them, the system says 16Gb but usuable is only 7.95Gb? any help will be appreciated