Prospect House, which cares for adults with learning disabilities and high-level autism, had developed a "closed culture", a report by the Care Quality Commission (CQC) said.
The venue, which is in Altofts, was previously placed in special measures in 2017 and 2019.
On both occasions it was upgraded to 'requires improvement' status one year later, though it has now been graded inadequate again.
The CQC said this "indicated a pattern of failure on behalf of the provider to ensure improvements were sustained".
Four Care Plus, which runs the venue, has been contacted for comment.
In their report following a visit made across four separate days in January and February this year, inspectors said the "provider had poor oversight of the service".
They reported conversations they'd had with the care home's workforce, some of whom said they had to come in on their days off because the service was short-staffed.
People within the home also suggested staffing levels were limiting day trips and outdoor activities, with one of the venue's six residents telling inspectors that having to stay inside "makes me bored stiff".
The report added: "The provider was reliant on the use of agency staff. We found that agency staff were supporting people without always knowing their needs and preferences.
The regulator also said there were "shortfalls" within the service's Covid testing regime, with some staff not receiving their results back after completing a test.
It was also suggested that risks had "not always been identified and managed safely", adding: "For example, one person was at risk of choking and they were left unsupported whilst eating their meal.
"The culture in the home was not positive and poor practice was not recognised."
However, inspectors did include positive feedback from relatives of people using the service, with one saying they saw long-standing staff members as "extended family".
The regulator added: "Some kind and caring interactions were seen. Staff did encourage some people to develop their independence."
As a result of being in special measures, the home is likely to be re-inspected within six months.
Local Democracy Reporting Service