Compatible ram don't work

I got a hp 6371it

link --> (http://h10025.www1.hp.com/ewfrf/wc/document?docname=c02054227&tmp_track_link=ot_faqs/top_issues/it_i...



I have this problem: I bought a kingston Dimm ValueRam 4GB DDR3 1333MHz Non-ECC CL9 (it is compatible with my mb according to hp website). I installed it on my pc and:
1) if I install just that ram it works fine
2) if I install that ram I bought and one ore both ram I had already installed in my pc it doens't work or it blocks on hp main screen or it appears the classic blue screen of hardware error and it never enter on windows (it asks for a system reinstallation)
what is the problem? should I do something else? I tried mounting rams in different slots, I tried everything

please help!!
 
Sound as though the ram you bought is not compatible with the existing ram. Is the ram you bought the same specs as the old ram?
 
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