In a certian category the comments seem to not work, in the category settings they are set to show comments . If in k2 settings I open up the comments to everyone then the fields to comment in appear. I want to limit it to registred users so I set it to registered only and they dissapear again in that category.

Thanks for any help on this issue.

Check comments setting under item view options for that category.  If that's not it, delete and re-add the category.