A report into HMP Dovegate also found high levels of violence, insufficient staffing, unjustified segregation and poor visiting arrangements.
It was, though, praised for good living conditions and resettlement work.
The Category B jail houses about 1,060 male prisoners and is run by the private firm Serco.
Chief Inspector of Prisons, Nick Hardwick said the jail's performance "had dipped" since its last inspection in 2013 under a new management team.
The report said there had been an influx of prisoners convicted of serious violent offences, and a large number of men who had to be separated from the rest for safety reasons, including gang affiliations.
The use of force by staff was higher than average, attendance at activities was poor, and skills provision was inconsistent.
Mr Hardwick said Dovegate had struggled to respond to the "destabilising impact" of new psychoactive substances.
"There remains much to do," he said.
Michael Spurr, from the National Offender Management Service, said: "The prison is working hard to tackle violence, and is working with Ofsted to improve education outcomes for prisoners."
HMP Dovegate houses a 200-bed therapeutic unit, which is subject to a separate inspection report.
The availability of new psychoactive substances has led to debt and bullying inside a Staffordshire prison, a report has found.