The death of a boy in a young offender institution (YOI) was the second since 2012 after staff delayed entering the cells during the night, a report says.
Daniel Adewole, 16, suffered "sudden unexpected death in epilepsy" at Cookham Wood in Kent in 2015.
An inquest heard that despite repeated attempts to rouse him officers did not enter the cell for 38 minutes.
The Prisons and Probation Ombudsman said concerns raised in 2012 after another death had not been addressed.
Following an independent investigation, ombudsman Nigel Newcomen said: "Daniel was the second boy to die at Cookham Wood since 2012.
"In our investigation into the previous death we were concerned that staff delayed going into the boy's cell during night state.
"We repeat our recommendation from that report."
'Lack of understanding'
The ombudsman reported on 4 July 2015, during a routine check, a member of staff found no cell-card to indicate anyone was in Daniel's cell and assumed it was empty.
He checked the roll board in the office and noted in front of Daniel's name were the letters "MED-E" (an abbreviation for Medical and Epilepsy).
The member of staff told the inquiry he did not have a good understanding of the medical code, and no-one had briefed him about Daniel's epilepsy or what to do in an emergency.
The ombudsman reported a "reluctance" to go in to Daniel's cell after staff got no response for him.
"In its response to one of our recommendations after the investigation into the previous death of a boy at Cookham Wood in 2012, the prison said, 'We will review instructions for procedures around opening of cell doors on night state to ensure they clearly remind staff of their responsibility to ensure there is no delay if there is potential for risk to the life of a child'.
"This investigation found little evidence that this had happened."