This problem occurred to me too. Sometimes the buttons fall off the "end" of
the toolbar, maybe if you don't use them. On the right end of the toolbar
there is a little black triangle and two chevrons like so:
V
and if you click on this the missing buttons are in a little menu which pops
up. If you use the button from the menu it gets put back onto the toolbar.
It's some kind of space saving scheme. I'm not too sure of the logic of how
it's decided which buttons get bumped off the toolbar though.