RAM Upgrade for HP laptop (Pavilion zv6000)?

I bought a 1 Gig Ram DDR, 333, CL2.5 which was the specs of the old one, it even had the same Manufaturers name. But the problem is, when i put it on and restart my laptop it doesnt work. SO i put the old one it and sure enough it works just right. Can anyone help with this problem? i sure appreciate it.