Slurpee machines difficult?

I like going to 7-Eleven for various reasons. They have a huge selection of on the go and junk food. The primary reason I go there though is to get a Coke Slurpee. This is apparently a problem more often than not though. At least half the time I go to a 7-Eleven the whole Slurpee machine or sometimes just the Coke portion is non-functional. It isn't usually the machine is off, it is that the Slurpee mix is not at the right consistency or isn't cold enough. This gets you liquid instead of the icy goodness you expect. Why is maintaining a Slurpee machine so difficult?