It was a spectacular success that researchers from the "4 Day Week Global" project presented in February. For half a year, around 60 British companies with more than 3,000 employees had tested the four-day week with full salary. And the results have been so good that 56 out of 61 employers wanted to keep the four-day week, at least for the time being.

As the surveys showed, the additional day off in the week was not only good for the vast majority of employees. The majority of employers were also enthusiastic. On average, the companies involved in the project performed even better than before, at least according to the evaluation of the researchers involved (read more about the results here). The captivating logic behind it: If the employees are feeling better mentally and physically, they can do more in less time and everyone benefits.

So why don't all companies immediately introduce the four-day week, which many employees in this country also dream of? This is probably due to the fact that the model does not work everywhere. First of all, it can be assumed that from the outset it was primarily companies that were realistically hoping for a positive effect that registered to take part in the British pilot project. And even with these, the plan sometimes went haywire.

The BBC has spoken to some of the companies involved, where the four-day-a-week project has failed - or at least is unsure if it will really prove viable in the long term. There's Mark Roderick, for example, who runs a wholesale building supplies business in Gloucester. His company Allcap employs 40 people. With the four-day week, Roderick actually wanted to reduce the high workload of his employees.

But the opposite happened: Although the Allcap employees only added an additional day off every other week, the workforce was overwhelmed with the model. "Instead of 10 normal working days, we found that staff had 9 extreme days - once they reached their scheduled day off, they were exhausted," Roderick told the BBC. "When we factored in vacation, sickness and caregiving responsibilities, we also had difficulty substituting for an employee on their day off."

The model was not practicable for Allcap to meet short-term deadlines and always have enough employees on site, since many jobs could not be done in the home office. At three out of five locations, Roderick ended the four-day experiment two months before the end of the originally planned period.

Other participants in the four-day experiment also complain about the price of work intensification. "Without a fifth day to catch up, there's generally more stress during the week to make up for a longer weekend," said Laura White, project manager at London-based charity Waterwise. Your company has extended the testing period beyond the six months to gain more long-term insights.

The four-day week can prove challenging, especially for customer-facing organizations that need to cover specific opening and office hours. Alison Dunn, head of the consumer advice center Citizens Advice in Gateshead, reports that she first had to hire three additional full-time employees in order to be able to take part in the four-day experiment with the entire workforce. In the project phase, it turned out that certain employees were able to complete their entire work very well in four days and even outperformed.

The contact center team, on the other hand, is simply too tied to fixed opening hours to be able to redistribute work flexibly, which had a negative impact. Because of this mixed result, Dunn initially extended the project through May. However, if ultimately not all teams benefit from it, they will probably abolish it again. "We have 220 employees: I can't imagine that we allow some to work a four-day week and not others," says Dunn.

For other companies, too, the project period of six months is still too short to make a final conclusion about the four-day week. It is also possible that the employees' motivation to give it their all for four days in order to earn an extra day off decreases over time. "We want to see how it feels over a full year and make sure productivity stays high," said Claire Daniels, head of digital marketing agency Trio Media. So far, however, she and her employees have been very impressed with the model.

Jo Burn-Russell found a pragmatic solution with her creative agency Amplitude. After the end of the trial period, everyone wanted to deal with the four-day week differently, she reports. Therefore, the whole thing is now optional in your company. "Rather than dictating a day off, it's better to let employees decide what's best for them. In an agile company with a team of 12, this works well." Good for those whose job allows such flexibility.

Sources: BBC / 4 Day Week Global

Read what a German top manager says about the four-day week here: Telefónica HR manager: "I'm skeptical about the four-day week"