Why is there always at least one broken down self-checkout lane at the grocery store?

Is it just where I shop or is it more common? The grocery store I shop at rarely has all four self-checkout lanes open at the same time.

Does yours? How many total self-checkout lanes does it have and how many are usually broken down? Why do you think these things always break down? Is it a mechanical or software issue usually? Do you know someone who works on them? Why can’t these machines be more reliable?