The Wales Audit Office (WAO) said the problem is still present four years after concerns were raised that staff were not coming forward after attacks. Reasons include staff accepting attacks as 'part of the job', and forms taking too long to complete.
The latest research shows things have improved, but the watchdog says there is still 'under-reporting' and staff are continuing to be 'reluctant to report all incidents of violence and aggression'.
Interviewed staff expressed concern over reporting incidents because of pressure at work, a lack of support from managers and acceptance of verbal abuse in A&E departments.
The report says the extent of the problem is going unrecognised as a result, and has called for more work to be done on guiding and training staff who need to physically restrain people.
It is hoped a memorandum of understanding between the Assembly Government and Crown Prosecution Service will reverse the low numbers of prosecutions, and another agreement is being sought with the police.