keep logging out

hi there
I am the admin of the blog where connect to the forum.

some user can use the blog without problem (create their own blog), some cannot log into the blog even so they are log in forum – once they go the blog, the blog ask them to log in (even so already done), then even they try again to log in the blog, its jump back to the forum.

strange thing is, its only apply to certain member, some working (even so, they are all in the same group of the user – registered user)

i checked all setting of the same group (registered user) in phpbb, all are the same, so its seems not related to the setting in phpbb.