Update Manager icon appears on the launcher when updates are available, clicking on it does not bring up the update-manager.

If its only a problem on that machine then its not a bug in the OS. I cannot produce the issue on my machine either. You would find better help with this in the forums or the answers part of launchpad. Closing as invalid.
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Same problem for me. The workaround is to open a terminal, type "pgrep update", kill update manager using the returned process id and then start Update Manager again.

Same problem for me. I have a laptop and desktop both
running Ubuntu 11.10 x86_64. The laptop is fine but the PC
has this bug.

Just right-click on the update-manager icon and close the application.
Then choose 'software up to date' in the main menu to relaunch.

Same problem for me. Also my laptop is fine but the PC has this bug.

Using alt-tab to select update manager doesn't open the window either.

Even reinstalling update-manager didn't help. I used this procedure to uninstall:

1.- sudo apt-get remove update-manager
2.- sudo rm -r /etc/update-manager
     sudo rm -r /usr/share/update-manager
     sudo rm -r /usr/lib/update-manager
     sudo rm -r /var/lib/update-manager
     sudo rm -r ~/.cache/update-manager-core
     sudo rm -r ~/.gconf/apps/update-manager
3.- sudo apt-get install update-manager

The only workarround is to close the icon in the unity bar and start update-manager from dash or the main menu.

Update-manager is version 1:0.156.14.4, as reported by "apt-cache policy update-manager"

Forgot to say that this problem is happening since I updated the PC to 11.04 and is still happening now that it has 12.04

I know tons of people with this bug.

I've experienced this but in my case the update manager window "opens", but the contents of the window don't draw. Killing update manager and restarting it seems to resolve it. (This is on 12.04, but I'm pretty sure I've had it occurring intermittently for at least a year or two.)

I experience both what saf described as well as what the original bug description is.
I can replicate on 11.10, 12.04 and 12.10 latest.

Happens to me as well, but just occasionally. It would be nice to know what some of us could do to help fixing this?
Is the problem with update-manager or Unity or both?

I think it's a duplicate of Bug #1003950 but I'm not marking it as duplicate because this bug it's not fixed (tested on fresh install of Ubuntu 13.10).

This seems to have been reported all over the place. I first commented on it in connection with Bug #1000500 for 12.10, which was later marked a duplicate of this bug, and have seen it elsewhere too. My report mentioned that I had experienced it "for a few releases now," and some here are also saying it goes back to 11.04. Well, like marcenuc, I can confirm that it's still happening for me now in 13.10, which I why I was led to search for it again and make this report. Should we take odds on for how many more releases it will persist? Like osmo-laitinen, I'd like to help fix this. Could it have anything to do with the --no-focus-on-map option it is launched with? Any experiments to suggest that might help locate the problem?

I am having this exact problem (so it would seem) with 12.04.

The Update Manager appears on the launcher, indicating that a number of updates are available. Clicking on the icon in the launcher does nothing.

However, following a time seen elsewhere, right clicking the icon and selecting close does allow me to find and run Update Manager from Dash Home.

I don't know if this helps but the same happens to me too when launching pidgin from the messaging menu at the top right. Pidgin window won't open and I would need to right-click, close and re-open.

Each new release leaves me hoping that this problem will get fixed. Alas, the problem still persists for 14.04.

Given that it has been reported and re-reported consistently since 11.04, and yet still has Undecided importance and is Unassigned, it makes me wonder about the mechanism that's used to move the process along. For example, is there someone looking at this that decides that it's too low priority to act on? Or is it not being noticed at all? Is there any way to find this out?

I personally have no experience with the relevant codebase, but if someone could help lower the barrier to entry by providing some pointers to where or what the issue might be, I'd be motivated to look into it, as I guess some others might be as well.

Confirming on 14.04 (which was updated from a previous Ubuntu release).
Somewhere (http://askubuntu.com/questions/305069/software-updater-window-not-displaying-though-open-in-13-04) it is suggested that this issue might be related to AMD proprietary graphic drivers, which I'm using indeed.

It's a usability issue that doesn't limit the functionality of a core package.

Issue on 14.04 still. Shows running in launcher and on alt+tab but you can't access it. Easily fixed by right clicking and choosing quit, but something like this will easily put off new adopters, especially since this is LTS. Just my two cents...

here is a work around that worked for me:
The reason the Update Manager would not appear for me was that the window was located off screen. I'm not sure why that is the case, but to see the window I just needed to Alt+Tab then Alt + F7 to move the window back onto the screen using the mouse.

Ah, this makes sense. I sometimes use a projector on my laptop, and, while the laptop is connected to the projector, the projector screen is mapped alongside my laptop screen (rather than shared) so that I have to drag windows off of my laptop screen for them to appear on the projector screen. After unplugging the projector cable, the virtual combined screen reverts to its original size, but perhaps something is not being reset at the same time, so that when the Update Manager window is later opened, it is opened into now non-existent screen space. That would explain what I'm seeing. Thanks, Bryan, for that observation and work-around.

I tried the approach of comment #22 but for me it does not work. Alt + F7 does not help showing up the window. Furthermore, I never use multiple screens, and I have the problem as well. Only with the Software Updater.