Summary:

I use Debian, with LXDE desktop environment. On the Panel that runs along the
bottom of the desktop, there are small program icons (such as for File Manager,
Konsole, Kate editor). When I single-click on them, the application should be
launched.

But sometimes when I click on these icons, NOTHING HAPPENS. Could someone tell
me why this is, and what do I do to get the icons to respond EVERY time.

Full Details:

After clicking the icon, I wait the usual few seconds for the application to
launch. Then I discover that it hasn't launched, and have to click the icon
again, and then it launches. This rigmarole is so frustrating and time-wasting.
(Not your fault of course). From what I can tell, it almost always launches on
the second click (ie, I don't have to try again after the second click).

Sometimes, when the mouse pointer is over the icon, the tooltip rectangle
appears (you know, saying what that icon launches). I can't be sure, but I've
got this feeling that the tooltip "bubble" is sometimes interfering with the
clicking on the icon, and stopping the click being registered.

Sometimes the application launches when I click the icon the first time, even
when the tooltip is on show.

My window manager is "metacity" (ver 2.34.1), and the Desktop Panel is "lxpanel"
(ver 0.5.9).

i guess you are aware that metacity is not lxde's standard window manager.
also i don't think that the tooltips are interfering.
we have to get to the bottom of "sometimes".
try to find out which command is executed when you press that launcher (this should be apparent when you attempt to edit it).
then open a terminal, and execute that command. can you reproduce the problem? if so, then we need the contents of that terminal window.

ondoho: Thanks for your quick reply.

No I didn't know that metacity isn't lxde's standard window manager. I'm newish
to Linux, and my system is the Knoppix Live DVD installed to hard drive, which
effectively becomes Debian. So I've got on my system whatever that installation
gave me - in this instance, LXDE Desktop Environment, and metacity window
manager.

I looked at http://lxlinux.com (LXDE-Openbox Guide) and see that LXDE's window
manager is Openbox, which isn't installed on my system. I wonder if using
Openbox instead of metacity would cure the no-response-to-click problem.
However, I have no experience with changing the window manager, so I wouldn't
know what to do.

I do appreciate what you're saying about running the command that is behind the
icon, in a terminal. However, I don't think it's the commands that are the
problem (although I'm not setting that statement in concrete).

The reason I say I don't think the commands are the problem, is that from time
to time I've launched some of the commands (that are behind those icons) from a
terminal (I use Konsole). And as far as I can recall, those commands always
launch first time.

Regarding the icons that sometimes don't respond to clicking, it seems to be
mainly those in the Application Launch Bar (of the Panel) that this applies to.

One reason why I have the feeling that the tooltip bubble might be interfering
with clicking these icons is because of the following. Sometimes, when you hover
over the icon, the bubble is about 1/8" to the right of the mouse pointer. But
sometimes the bubble is VERY close to the pointer. I THINK this is when you
happen to move the mouse pointer slightly, AFTER the bubble appears.

I had the idea that it might be this situation (mouse pointer very close to the
bubble) that causes the click not to respond. But I've just been testing
this out, and so far the icon responds to the click first time.

Indeed, generally at the moment, the non-response isn't happening very often.
Sometimes it behaves properly for entire sessions that I'm on the computer (laptop). I
seem to think that at one time, the problem was happening more frequently. Also, as a
general observation, the non-response seems to occur randomly.

I see that some of the commands behind the icons have a percent parameter with
them, eg "pcmanfm %U", "kate -b %U". So I was wondering if it might be the
percent parameter that sometimes causes the non-response. I don't know what that
parameter is for.

I envy your ability to write so briefly.

no, it wouldn't.

no, they don't.

based on your information, i'd say it's a user interface design flaw and nothing else.
try different gtk themes, they are responsible for tooltip design, and some might choose to place them further away from your pointer.
you can also try to change the position of your panel.