Dell RAM Upgraded Issue

I have dell vostro 3358 laptop. Recently i upgraded my old hdd to sdd and replaced cd drive to hdd via cady and today i im trying to upgrading my ram i brought Hynix sk 8gb 2rx8 Pc3l-12800s 1600mhz... When i put both ram my old one and new one then my laptop beeping only if i put only one ram 4/8 its working fine.. Why?