Usually, low density will work in high density motherboard, but not the other way around. First time I heard of a laptop problem. It could be that the eBay vendor sold you mislabeled RAM. Whether intentional or not, it's always best to stay away from eBay, because it's mostly close-out or grey market junk that can't be sold anywhere else. They could even be Chinese knockoffs of name brand devices.
Unless you can buy the modules locally, you can't go wrong with Crucial, their database seems to always be 100% correct. BTW, they are a division of Micron, who manufactures the chips & the modules. Micron now owns Lexar.
Says this on one of the web pages: © 2011 Micron Technology, Inc. All rights reserved.