Following legislation passed in 2016, all deaths in custody must be the subject of a Fatal Accident Inquiry (FAI).
But a backlog means the families of prisoners who died as long ago as 2015 are still waiting for the outcome of an inquiry.
Dionne Kennedy, 19, took her own life at Cornton Vale in 2014 after being held on remand for breach of the peace despite suffering from mental health issues and having a history of self-harm.
Following a FAI held last year, Sheriff William Gilchrist published his determination last month and was unable to identify any precautions which could have been taken which “might realistically have resulted in the death being avoided”.
However, he said he shared the family’s concern at the length of time it had taken to hold the inquiry.
Dionne was one of a number of prisoners who died in 2014 or 2015 where relatives have had to wait years to receive a determination on the cause of death.
The issue came to the fore last year following the deaths of Katie Allan, 21, and William Lindsay, 16, at Polmont Young Offenders’ Institution.
William killed himself within 48 hours of being remanded there despite being flagged as a suicide risk.
The lawyer representing the two families has suggested there is a “spiralling epidemic” of suicides in custody.
A spokesman for the charity Families Outside, which provides support for those with loved ones behind bars, said: “We really want to ensure that families get closure and there’s a real need to speed up the response.
“It’s putting a lot of emotional pressure on families and we will continue to provide support for them.”
In all cases, a death certificate is provided to the family of the deceased, often far in advance of the FAI taking place.
The Crown Office and Procurator Fiscal Service (COPFS) admitted that the time taken to investigate some deaths had been “too long”.
A spokesman said: “COPFS has recently increased the resource available to the Scottish Fatalities Investigation Unit with a view to reducing the time required to complete complex death investigations and improving the provision of information both to families and next of kin.
“In addition, COPFS has revised the way the progress of all death investigations is monitored to ensure that they are completed as efficiently as possible.”
He added: “These measures represent a commitment to achieving a significant improvement in the service delivered by the Procurator Fiscal in this important area of work.”