Recently upon loading the system monitor in Ubuntu the memory column has been conspicuously absent. Some research revealed this to be a fairly old bug with the program that thankfully has a nice and simple fix.
This launchpad bug report deals with the issue, and it's the reply by Doc along with those that follow that reveal a solution, as well as suggestions upon how to deal with the various ways the problem manifests itself.
First load Gnome configuration editor:
gconf-editorThen navigate using the following steps as detailed by Doc:
apps -> procman -> proctree -> columns_order --> Add n.15 if missing, and if present, reset its position to follow the numerical order properly.
apps -> procman -> proctree -> col_15_visible --> Set to true
Hopefully that has fixed the issue!