When I first upgrading computer parts, I figured out that sometimes parts aren't all the way in even though it feels like you couldn't push anymore without breaking it.
Especially with RAM. With RAM, make sure that the RAM stick is far enough in so that the little white arms that click to hold it in place are all the way vertical. But I wouldn't just try to force it in if that's the case; try taking it out and putting it in at a better angle.
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