Laptop detects additional Ram but it doesn't show it further in bios or Windows

Feb 21, 2018
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I have read multiple threads similar but none were exactly like my problem.
My laptop is Dell Inspiron 7559, core i7 6700 hq 8 GB Ram nvidia 960m, I recently bought another 8 GB of ram same factory same details almost identical to the on i had. I put it on second slot then turned on the laptop and I received the typical bios message where it says the amount of RAM in this computer has changed. I continued to the windows but there was no second ram only 8 GB... I checked bios again it didn't show the RAM, I put ram on other laptop it worked fine then I tried another 4 GB ram on my own laptop same thing happened. I would say my laptop's hardware (Ram slot) is defect but I am suspicious that the slot doesn't work how does it recognize the ram change in the beginning.
I would appreciate any kind of help. many thanks in advance.
 
Feb 21, 2018
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I opened u my laptop and it took me an hour :D but removing the Cmos battery didn't do any thing... I reseted Bios from bios menu, that didn't do any thing as well. oh btw i have tried either slots.. they respond only to my RAM.
So back to the first square... any other suggestions ?