is it the right kind or RAM. there are many different types. that would be the most logical problem.
also, do you know if it needs to be paired or not? depending on the type of RAM and your motherboard, sometimes you need to have 2 identical pieces of RAM for it to work correctly. more commonly you don't and can mismatch it, but that could be something to look into.
are you sure the RAM slots are working properly? you said the computer was old and emachines are exactly the pinnacle of hardware reliability, so it might just be fucked.
the only other thing i can think of off the top of my head, is if there is some sort of setting, like in the BIOS or something that could be restricting it.
where do you see the amount of RAM and how do you know its wrong. are you going by the amount in the count up on startup or are you going by what it says in the OS. there could be discrepancies between them, if its not working right.