Memory problem after upgrade to El Capitan I have an iMac 27 inch ( Late 2013 ) system.It has 8 GB RAM. Today i upgraded to El Captain from Yosemite. El Captain installed successfully. Now it is showing 4 GB Memory. I dont know what happen to remaining 4 GB Memory.
I followed the comments and re-inserted the RAM.  I rebooted and the system showed both memory ports full with full RAM.  I even switched the RAM to see if that would reveal any new information.