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Assuming that you put the batteries in correctly (I don't mean anything by that, yes, it happens) then you may have some corrosion on the contacts. To take care of that, remove the battery pack assembly and then look inside the shell. Remove the little white lock bar, then slide the PCB out of the shell. On the PCB, you'll see the contact points (where the battery pack contacts the PCB) and you can clean those up with an eraser (an ink eraser works best, but you can use a pencil eraser) until they are nearly shiny.
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