i try to upgrade ram on my aluminium mac,it takes 16 gig

when i insert all 4 sticks 4 gig each it beeps and wont start, then i put 14 gig ( 3 4 gig sticks an one 2 gig stick) and it works,i sweep the rams,i test all rams and all work,what could be wrong when i insert al 4 4 gig sticks?? its a 3.06 ghz intel core duo 10.6.8