|Check in BIOS how much memory it shows there.|
Run Memtest86 via a bootable CD to both see how much memory it shows and if it finds any errors.
Open the bottom access panel and reseat your memory modules and see if that helps.
If this does not help and the laptop is under warranty, contact HP.
Even it it is not under warranty, the memory may be under warranty by the vendor depending on the vendor and their agreements with HP.
Post back results.
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