In ubuntu 12.04, my system-update goes like this: I click open the update manager (roughly once in two weeks) and it always says at first that there are no updates for my computer. However next I click the "check" button and the "updating cache" process starts and when this is finished I am always told that there are some updates for my computer. My doubt is why the update manager is unable to detect that there are some updates until I use "check" option in the manager? Is there something wrong with my configuration? In 10.04 I used to receive the updates automatically from ubuntu. But now I have to do it manually. Why?

Also what is actually done in the process of "cache update"?