Not getting a good enough apology when things go wrong is the most common complaint escalated by NHS patients in England, figures show.
It was the reason behind 34% of cases investigated by the Parliamentary and Health Service Ombudsman in 2014-15.
Errors in diagnosing conditions, poor treatment and a lack of communication were also among the top reasons acute hospital trusts were referred.
The organisation upheld 726 complaints out of the 1,652 it investigated.
The PHSO is the final port of call for patients in England who are unhappy with a hospital's original handling of their complaint.
The ombudsman has itself been criticised in recent years for not doing its job well enough by investigating too few cases and dragging its heels over decisions.
Its latest report shows it has investigated more complaints than last year - 1,652 in 2014-15, compared with 852 in 2013-14.
The investigations resulted in 36% of cases about the NHS being upheld, alongside 44% about acute hospital trusts.
Parliamentary and Health Service Ombudsman Julie Mellor said: "We know that there are many factors that influence the number of complaints hospitals receive, such as organisational size, demographics and whether they actively encourage feedback from patients.
"I strongly believe that NHS leaders should welcome feedback from patients and recognise the opportunities that good complaint handling offers to improve the services they provide.
"We are publishing this data to help hospital trusts identify problems and take action to ensure trust in the healthcare system remains high."