Hello and welcome.
Possibly incompatible RAM or the RAM is corrupt.
Run this scanner on the machine and it will tell you what is compatible and how much can be added and how much each slot can hold.
RAM Memory Upgrade: Dell, Mac, Apple, HP, Compaq. USB drives, flash cards, SSD at Crucial.com
The possible issue is you say older machine.The machine may only be able to support machine sticks of RAM.As in 2 x 256 MB of same speed etc.
The scanner will tell you.