1st, they should be in slots #1 & #3 or #2 & #4... 2nd, Dell computers have issues with the CL of RAM... I believe the OEM are CL 11 RAM and they like the slower CL 11 RAM and not CL 9 and smaller. you may want to try the original ram in 1 and 3 and new RAM on 2 and 4 and see if it sees all the...