For a lot of people, dessert isn’t an everyday thing, so when you finally are in the mood to treat yourself, the last thing you want to be told is that the one thing you’re hankering for is not available. Yet, for years, ardent fans of McDonald’s McFlurry have complained about being denied their favorite frozen treat.

The constant lack of ice cream availability has predictably spurred an onslaught of social media complaints (see just a few recent examples in the sidebar) and created even more questions for the fast food giant.

For those not familiar, the McFlurry is McDonald’s take on Dairy Queen’s Blizzard. McDonald’s, which also serves sundaes and ice cream cones, offers customers the choice of adding either M&Ms or Oreo cookie pieces to their dessert.

But the biggest ingredient for the dish is ice cream, and as many customers have shared on Twitter, many McDonald’s don’t have any because machines are constantly broken.

McDonald’s lack of ice cream — and therefore McFlurry treats — isn’t a recent problem. Five years ago, McFlurry fans gathered on reddit to gripe about their inability to get their desired dessert, and to hypothesize on the possible reasons for McDonald’s ice cream-related difficulties.

“Someone who works at McDonalds and can explain why the ice cream machine is almost always down,” reddit user balomus posted in 2012. “I am legitimately curious as to why this is always the case at McDonalds. Is it poor equipment? Do they have a required cleaning procedure that takes an extended period of time and must be performed multiple times a day?”

A poster claiming to be a former McDonald’s employee said part of the issue is indeed the cleaning of the machine, but also lazy workers.

“Truth is, the machine is terrible,” the redditor said. “It is dirty all the time, and after 9, most people get lazy and tear it down early.”

A spokesperson for McDonald’s tells the Wall Street Journal that the nightly cleaning of the machines, which coincides with customers’ desire for the treat, definitely plays a role.

“We regularly service our soft-serve equipment during off-peak hours,” the spokesperson said. “Customers who come in during that time may encounter a longer wait time or soft-serve dessert unavailability.”

Another reddit user, who claims to be a former employee of the Golden Arches, notes that the end of the night is also when employees turn on the “heat mode,” which cleans the machine.

During this time, the contents of the machine are heated up, meaning the ice cream melts, placing the machine “out of order” until the mode is done.

The former employee also claims that the machines are used so frequently and a large volume of ice cream passes through, leading the machines to “break all the time.”

“Even if a part doesn’t actually break, the machine cannot keep up with the volume, and the ice cream becomes too soft or runny to make anything with it,” the poster writes. “It has a hard time keeping the mix cold and turning it into soft serve.”

Others note that the cost of the machine — and its parts — are so expensive that some franchisees don’t want to spend the money to fix the machines.

After all, ice cream isn’t McDonald’s core menu item, so a machine may be able to sit idle for some time without doing too much damage to the business.

The WSJ story points to a survey from 2000 — about two years after the dessert was launched — which found that 25% of McDonald’s restaurants had ice cream machines that were out of order, giving an indication of dessert’s low priority for some franchisees.

As for the customers who have complained about the broken machines on Twitter, the company has replied, telling guest that it strives “to ensure customers have an enjoyable experience every time they visit” and that it take appropriate steps to handle their concerns.

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