Cannot bring back some minimized windows

[LutraMan]

I have a new installation of Ubuntu 12.04 that came with Unity.

On the left panel I have links for apps. When I start the app, it gets a small triangle to its left indicating that it's activated.
Some of the apps I've started have changed the triangle to an empty triangle, they are still on, but clicking on the icon now does nothing. If I haven't minimized them I can still get to them by moving the other windows to see them beneath them, but if I'd already minimized them, I have no idea how to get them back.

Can anyone explain this behaviour?

[JohnnyComeL8ly]

I haven't used Unity for a while, but you can try [Alt + Tab] to switch between open windows. If you know the name of the app that is acting up, for example: "crazyapp2;" then try this in a Terminal [Ctrl + Alt + T]
pkill crazyapp2 Now, if you don't know the name of the app, you can try
ps -A in the terminal and get a good guess as to which one(s) on the list you would like to kill. For example:

me@mymachine: ~$ ps -A
PID TTY TIME CMD
1 ? 00:00:03 init
2 ? 00:00:00 kthreadd
3 ? 00:00:52 ksoftirqd/0
5 ? 00:00:00 kworker/0:0H
7 ? 00:00:14 rcu_sched
8 ? 00:00:00 rcu_bh
9 ? 00:00:00 migration/0
10 ? 00:00:01 watchdog/0
11 ? 00:00:01 watchdog/1
12 ? 00:00:00 migration/1
13 ? 00:22:07 ksoftirqd/1
15 ? 00:00:00 kworker/1:0H
16 ? 00:00:00 khelper
17 ? 00:00:00 kdevtmpfs
18 ? 00:00:00 netns
19 ? 00:00:00 writeback
20 ? 00:00:00 kintegrityd
21 ? 00:00:00 bioset
22 ? 00:00:00 kworker/u9:0
23 ? 00:00:00 kblockd
24 ? 00:00:00 ata_sff
25 ? 00:00:00 khubd
26 ? 00:00:00 md
27 ? 00:00:00 devfreq_wq
30 ? 00:00:00 khungtaskd
31 ? 00:00:02 kswapd0
32 ? 00:00:00 ksmd
33 ? 00:00:01 khugepaged
34 ? 00:00:00 fsnotify_mark
35 ? 00:00:00 ecryptfs-kthrea
36 ? 00:00:00 crypto
48 ? 00:00:00 kthrotld
51 ? 00:00:00 scsi_eh_0
52 ? 00:00:00 scsi_eh_1
53 ? 00:00:00 scsi_eh_2
54 ? 00:00:00 scsi_eh_3
77 ? 00:00:00 deferwq
78 ? 00:00:00 charger_manager
129 ? 00:00:00 scsi_eh_4
130 ? 00:00:07 usb-storage
131 ? 00:00:00 kpsmoused
139 ? 00:00:02 jbd2/sda1-8
140 ? 00:00:00 ext4-rsv-conver
261 ? 00:00:00 upstart-udev-br
267 ? 00:00:00 systemd-udevd
299 ? 00:00:00 cfg80211
315 ? 00:00:00 kworker/u9:1
442 ? 00:00:00 upstart-file-br
513 ? 00:00:01 rsyslogd
555 ? 00:00:00 upstart-socket-
634 ? 00:00:52 dbus-daemon
672 ? 00:00:00 ModemManager
692 ? 00:00:00 bluetoothd
701 ? 00:00:00 systemd-logind
726 ? 00:00:00 cupsd
727 ? 00:00:00 krfcommd
782 ? 00:01:22 NetworkManager
833 ? 00:00:00 polkitd
891 tty4 00:00:00 getty
896 tty5 00:00:00 getty
900 tty2 00:00:00 getty
901 tty3 00:00:00 getty
905 tty6 00:00:00 getty
937 ? 00:00:00 cron
1000 ? 00:00:34 irqbalance
1044 ? 00:00:00 lightdm
1049 ? 00:00:20 wpa_supplicant
1078 tty7 00:23:38 Xorg
1081 ? 00:00:00 accounts-daemon
1105 ? 00:00:01 whoopsie
1121 ? 00:04:50 mysqld
1138 ? 00:00:03 dnsmasq
1307 ? 00:00:13 apache2
1354 ? 00:00:00 kauditd
1551 ? 00:00:00 timidity
1557 tty1 00:00:00 getty
1606 ? 00:00:00 lightdm
1618 ? 00:00:00 init
1677 ? 00:00:00 dbus-launch
1679 ? 00:00:00 dbus-daemon
1691 ? 00:00:07 dbus-daemon
1694 ? 00:00:00 ssh-agent
1699 ? 00:00:00 upstart-event-b
1720 ? 00:01:15 ibus-daemon
1733 ? 00:00:00 upstart-file-br
1734 ? 00:00:01 lxsession
1737 ? 00:00:04 upstart-dbus-br
1739 ? 00:00:08 upstart-dbus-br
1742 ? 00:00:00 gvfsd
1747 ? 00:00:00 gvfsd-fuse
1756 ? 00:00:00 ibus-dconf
1757 ? 00:00:31 ibus-ui-gtk3
1759 ? 00:00:26 ibus-x11
1763 ? 00:00:00 at-spi-bus-laun
1767 ? 00:00:00 dbus-daemon
1770 ? 00:00:00 at-spi2-registr
1782 ? 00:00:16 ibus-engine-sim
1797 ? 00:00:22 openbox
1800 ? 00:00:54 lxpanel
1801 ? 00:00:36 pcmanfm
1808 ? 00:00:00 ssh-agent
1823 ? 00:00:00 menu-cached
1830 ? 00:00:02 gvfs-udisks2-vo
1836 ? 00:01:04 udisksd
1846 ? 00:00:00 gvfs-afc-volume
1851 ? 00:00:00 gvfs-gphoto2-vo
1855 ? 00:00:00 gvfs-mtp-volume
1865 ? 00:05:59 pulseaudio
1867 ? 00:00:03 rtkit-daemon
1871 ? 00:00:00 gconfd-2
1874 ? 00:00:00 gvfsd-trash
1890 ? 00:00:00 gvfsd-metadata
2008 ? 00:00:00 xfconfd
2758 ? 00:00:00 gvfsd-http
3190 ? 00:00:00 console-kit-dae
4035 ? 00:01:15 nm-applet
4053 ? 00:00:00 gnome-keyring-d
6380 ? 00:00:00 upowerd
7080 ? 00:00:00 kworker/1:0
7249 ? 00:00:07 gnome-terminal
7253 ? 00:00:00 gnome-pty-helpe
7254 pts/1 00:00:00 bash
8035 pts/1 00:00:00 nano
10321 ? 00:00:02 kworker/1:2
10344 ? 00:00:00 dhclient
10452 ? 00:00:06 ntpd
10481 ? 00:00:07 kworker/0:0
12165 ? 00:00:00 apache2
12166 ? 00:00:00 apache2
12167 ? 00:00:00 apache2
12168 ? 00:00:00 apache2
12169 ? 00:00:00 apache2
12406 ? 00:00:00 dconf-service
12410 ? 01:44:38 firefox
12449 ? 00:00:00 apache2
12667 ? 00:00:02 kworker/u8:3
12690 pts/3 00:00:00 bash
12718 ? 00:00:02 kworker/u8:0
12721 ? 00:00:00 kworker/0:1
12736 ? 00:00:01 kworker/u8:1
12744 ? 00:00:00 kworker/u8:2
12747 ? 01:14:57 crazyapp2 <fictitious: only for example>
12758 pts/3 00:00:00 ps
You can now also use
kill 12747 instead of
pkill crazyapp2 because you know the PID of the app.

I hope this helps you; if not, ask me more.

P.S. I had the same problem in Unity, but now I use LXDE because my olde PC won't keep up (I can't say what your problem is for sure).