How do you "lose" an airplane?

I admittedly don't know a lot about this kind of thing, so I might be making myself look foolish with this post. However, I will push forward. This is not the first airplane that has been "lost." I just don't understand how with modern technology and tracking systems, that it is possible to completely lose an airplane and have no idea where it is. Don't people put freaking tracking devices in their dogs and cats to be able to find them? How can lose an entire plane worth millions of dollars and holding dozens or hundreds of people?
Again, like I said, I am sure I am missing something here and am stupid, but anyway it is what it is.