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Would your computer actually be a hp pavilion dv6? If yes, the Crucial Memory Advisor
shows it should be able to handle 8GB RAM. But it's recommended that the modules be a matched pair. If the original stick was brand "ABC" and the new stick is brand "XYZ", there could be some compatibility issues.
You could check if the new stick is defective by putting it in slot 1 and see if the computer recognizes it. And you could also try swapping the sticks into the two slots. In other words, if you had stick 1 in slot 1, stick 2 in slot 2, try reversing them. Stick 2 in slot 1 and stick 1 in slot 2.
It might also help if you posted a screenshot of your resource monitor, memory tab. Screenshots and Files - Upload and Post in Seven Forums Resource Monitor