Scotland's chief inspector of prisons has called for more than 70 improvements to the running of the Polmont young offenders institution.
Polmont, near Falkirk, is the largest institution of its kind in Britain and holds adults aged 16 to 21 who are awaiting trial and serving sentences.
In his latest report Brigadier Hugh Monro describes the unit as modern, clean, well-maintained and with good facilities but said young offenders need to be given more activities to get them out of their cells.
The inspector cited high management turnover as a problem and said stability was the key if progress was to be made.
He singled out Blair House where under 18s are held and which has seen five managers since it opened in early 2010.
Brigadier Monro said in his report: "Young offenders are not sufficiently encouraged to take part in purposeful activity. There are insufficient opportunities to engage in purposeful activities. This is especially the case in the evenings and at weekends.
"Where opportunities to engage in purposeful activity are available these are not fully utilised, with high numbers of young offenders remaining in their cells during the day."
The inspector added: "Where there is criticism in this report, much of it relates to this spate of managerial moves. It is important that HMYOI Polmont now has a period of stability in order to assimilate the recommendations of this report and to instigate change."
Polmont, with a capacity of 760, was holding 630 young offenders at the time of the inspection.carried out from October 22-30 last year. The site has been redeveloped since inspectors last visited in May 2007.
Among his recommendations, Brigadier Monro said Polmont staff should introduce an appropriate policy to combat and respond to bullying and should ensure that rotas always include sufficient members of staff trained in control and restraint techniques.