The trust said a new computer system had caused delays
The head of South Central Ambulance Service in Berkshire has insisted its response times are improving despite recent criticism.
It emerged on Tuesday the service had to issue a formal apology to a stroke victim in Wokingham who had to wait for more than an hour for paramedics.
It was also rated as "weak" by a health watchdog for failing to meet targets.
Mark Ainsworth said an ambulance and staff shortage had now been dealt with and they were hitting response targets.
Ken Lincoln was taken ill on 9 September. His wife called an out-of-hours doctor service, which in turn called for an ambulance.
But Mrs Lincoln had to call 999 a further three times before paramedics arrived, one hour and 20 minutes later.
Speaking on BBC Radio Berkshire, divisional director Mr Ainsworth said the delay in his case was down to staff getting used to a new computer system.
"The call was categorised incorrectly," he said.
"The call originally came from the doctor and was put in as a one hour call for us to attend.
Mark Ainsworth said response times were on target in Berkshire
"Unfortunately when it got to the one hour, we were very busy with 999 calls which take priority because they appeared to be more life-threatening.
"We have apologised to Mr Lincoln and we have carried out an investigation. We have put hands up, we have made a mistake with this patient."
An annual report from the independent Care Quality Commission published on 15 October put the South Central Ambulance Service among the three worst in England.
The report said the proportion of life-threatening calls not reached by the service within eight minutes was too high.
Mr Ainsworth said: "The main issue has been resourcing.
"We've had a number of staff vacancies and we've now recruited at lot of staff into the service across the whole trust."
The service provides 999 cover for Hampshire, Berkshire, Oxfordshire and Buckinghamshire.