It may be simply that they lack the confidence to voice their concerns or find their school unapproachable, new research has found.
Schools are "quietening" the parental voice by throwing up institutional barriers. This can prevent parents becoming more involved in policy and academic issues, says the research, funded by the Economic and Social Research Council.
A massive cultural change by schools and parents will be needed if meaningful home-school partnerships are to develop, as ministers want.
The two-year project confirms previous findings that most parents are inteested in their children's educational development, even if they don't always attend parents' evenings and other meetings.
Many are also interested in school issues, such as homework policies and curriculum matters.
Professional, better-educated and wealthier parents are more likely to get involved. The silence of others can mask real concerns, but such parents are often cynical about whether it will make a difference if they do voice their concerns.
Professor Stewart Ranson, of Birmingham University said that there was a need to transform the "deep-seated culture" of deference that stopped many parents speaking their minds.