Mac App Store doesn't show OS X updates

Question

Since I upgraded to Mountain Lion in July, the Mac App Store has stopped showing updates for OS X. It shows normal app updates like Evernote but it didn't notify me for the 10.8.1, the 10.8.2 and the recent Supplemental Update. This happens even after days that the updates are released. I discovered that there were updates available from news sites.
I contacted Apple Support but I was quite rudely shut up. The guy at the phone told me that "this is normal" and that Apple "is not obliged" to issue a notification for all the system updates. I told him that all my Mac-owners friends could download the updates from the App Store but he told me that "they were lying" to me. I was so aghast at his response that I didn't started laughing in his face as he probably deserved. I will find a way to get back to him via an official complaint to his superiors, that's for sure.
Can anyone help me with the Mac App Store issue, anyway? Currently I'm forced to download the updates manually from Apple's site.
Thanks in advance for the support!

Answer 1

It's possible you have a problem with your system. I've had this same issue with Lion. Eventually, I bit the bullet and did a scratch reinstallation which seemed to fix things. But there may have been less invasive solutions I just did not think of or find. So you might start with the following:
Boot into Safe Mode then restart normally.
Download a utility such as Mountain Lion Cache Cleaner and use to clear out user, system, and font caches. Restart is immediately required.
Reinstall OS X.
Reinstalling Lion/Mountain Lion Without Erasing the Drive
Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the main menu. Repair the Hard Drive and Permissions as follows.
When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list.  In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive.  If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the main menu.
Reinstall Lion/Mountain Lion: Select Reinstall Lion/Mountain Lion and click on the Continue button.
Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.