the tab "Post new thread" that regularly appears on the top right corner , has been disappeared for some users.....
it is very strange behavior , because one of the users has administrator role and has the same problem ....
Try on a default style to check if everything is ok.
Try Analyzing a user(Admin CP -> Users -> Analyze Permissions) who is having problems to see where the permissions could be messed up.
Make sure there's not specific node permissions on these nodes where you're having trouble.