Only default links working

Hi

I am getting some strange behavior with my new installation, with only “Default” links working. I normally prefer changing settings to “post” name, but only 3 or so pages could be set to this.
All other pages, whatever I do, whether I first set all to default & then change to post name, all except the 3 are not found!
Even under post name settings, newly created pages are not found.
What can I do to get them post name permalink settings for all pages working?

Thank you in advance