Apps Icons not appearing
I can think of three (well, sort of) potential solutions to this. You do not need to try these in any particular order - try one first and if the issue returns, then try the other. Option 1 requires entering commands in Terminal, Option 2 doesn't but sometimes only works temporarily. And the third option is doing both Option 1 and 2.

Option 1: Reset icon cache

If you tried the second solution first and it doesn't work (or it only works temporarily), you can try resetting the icon cache.


Launch Terminal (usually found in the Applications > Utilities folder)
Enter the following commands, one at a time:


sudo find /private/var/folders/ -name com.apple.dock.iconcache -exec rm {} \;
sudo find /private/var/folders/ -name com.apple.iconservices -exec rm -rf {} \;
sudo rm -rf /Library/Caches/com.apple.iconservices.store
killall Dock


Option 2: Reset and restart the Dock

This remedy is to reset and restart the Dock. However, in doing so, you will need to re-add any non-default icons to your Dock. (I usually find it's a good opportunity for people to start afresh and only add things as they actually use them).

So, if your solution doesn't pan out, you can reset and restart the Dock as follows:


Go to Finder
Press and hold the option key down
Now go to the Go menu
Select the Library option to open the User's Library folder. You can let go of the option key now.
Go to the Preferences folder (i.e. within the Library folder)
Find and move the com.apple.dock.plist file to the Trash
Now open Terminal (usually located in Applications > Utilities)
Enter this command killall -KILL Dock
Press enter


This should resolve the issue - but remember you'll need to re-add some apps back to the Dock.

Option 3

You may find you need to do both options 1 and 2.

Let me know how you go.