The ombudsman is the final port of call for patients in England who are unhappy with a hospital's original handling of their complaint.
In 2012-13, the ombudsman received 16,000 complaints about the NHS. It took a closer look at 3,770 of these and investigated 377, which was a 5% drop on the previous year.
Sir Brian Jarman, Emeritus Professor of medicine at Imperial College London, said he was horrified when he looked at the number of complaints the ombudsman was investigating.
"The rest of them are effectively thrown into an electronic waste paper basket," he said. "I was amazed that they didn't investigate more. We should use complaints as gold dust."
Changing the way the PHSO works
After she was appointed head of the PHSO in 2012, Dame Julie Mellor decided to increase the number of investigations to around 4,000 a year but without any increase to the Ombudsman's £33m budget.
She said it was possible to work with their existing finances because they had changed the way they work.
But Charlotte Leslie MP, a Conservative member of the Commons Health Select Committee is sceptical this can be achieved.
"The more apologetic they are and the more they voice their failures, the more sceptical we become," she said. "We have heard it all before, we actually need to see results."
As part of his report into high-death rates, Sir Bruce Keogh, the medical director of NHS England, believes the current system needs to change as the ombudsman is struggling to cope with the demands placed on it by patients.
"If they don't get a decent response from the hospital they have to go directly to the ombudsman and the ombudsman in my view doesn't have enough capacity for that," he said.
Prior to 2009, hospital complaints could be taken up with the regulator, the Care Quality Commission (CQC), before going to the health ombudsman.