Optiplex 780 - DDR3 from kingston not supported

Hello,
I don't understand why my Optiplex 780 DT refuses to recognise DDR3 sold by the computer shop . Currently my Optiplex is running perfectly with 2 DIMM of 2 GB each from Hynix type HMT325U6CFR8C-H9 located in DIMM slots 1 & 2. I've tried to replace them with 4 GB DIMM from Kingston type KVR13N9S8/4 but the computer beeps at power on. Since both types are DDR3-1333 9-9-9 unbuffered non ECC I don't understand why those from Kingston are not recognised.

Any Idea?

Regards;

AN