If it is a commandline application then it will not appear in the menus.
Otherwise it will, but it is not always where you might expect it. I have
seen the rare instance of an application that does not appear at all, but
it has been a long time.

You can right click on the KDE icon (start button) and choose Edit
Applications to create your own launcher. Sometimes they are actually there
but you need to check a box to make them appear or you may have to choose
an icon and type the launch command to make a launcher. Items can be
dragged and dropped from one group to another, as well.

Most problems of this sort are due to commandline applications, items
installed outside of the repos and applications from other desktop
environments which may not appear where you might expect them.

My problem is that I have so many apps installed that they are hard to
find.