Constables returning body parts from unexplained deaths to relatives are still uncertain about their role, inspectors said.
A new process was adopted this year and some officers are still to receive training, independent experts said, but added that the PSNI identified and sent samples to loved ones quickly and with sensitivity.
An inspection was ordered by Stormont Justice Minister David Ford after police and the police ombudman's office kept human tissue belonging to more than 60 people without their families' knowledge. It was part of a UK-wide review after concerns were expressed about poor standards in hospital mortuaries.
Her Majesty's Inspectorate of Constabulary (HMIC) said: "As the process created by the inspected organisations was only adopted in 2013, HMIC found that some police constables are still uncertain about their role regarding the return or disposal of human tissue samples."
During a post-mortem examination samples of human tissue may be taken and retained for analysis by medical specialists or, if the death is suspicious, referred to Forensic Science Northern Ireland. This work can take weeks to complete, meaning fragments like hair cuttings cannot be returned in time for normal burial.
The PSNI has apologised for retaining samples belonging to 63 people - included bones and organs - between 1960 and 2005.
The force confirmed that some of the cases were deaths related to the conflict. The police ombudman's office also revealed it had kept body parts of some victims of unexplained deaths between 2001 to 2006 and apologised.
The cases emerged as part of a UK-wide audit of all police forces.
The PSNI has involved police trainers at the outset to address recommendations made by the Association of Chief Police Officers (Acpo) and will deliver training to constables later this year, the inspectorate said. "However, no date has been set for the training," the HMIC report added.
Inspectors recognised that the force and the ombudsman's office had developed closer working relations with the coroner's service and state pathologist. Their report added: "The PSNI led the changes in working practices with their partners and a considerable amount of progress has been achieved in a relatively short time frame."