MyBB Community Forums

some topics can't shown on visitor's side which shown image

You have to explain this more. I don't understand what you mean.

members of forum can view all threads but visitors can view only one thread.

What plugins do you have??

there's no plugin

Make sure at the sorting options at the bottom of the forum, you choose to view all threads from the beginning

See attachment

activated "From the beggining" option but still not solved problem.

Sample : http://www.mybbturkiye.com/ana-sayfa-f-89.html

what about this setting ?
[attachment=11544]

setted like this

What about the 'Default View Options' in the settings for that forum, what are they set at??