Subdomain data can't sent to subdomain/somthing

I have made a subdomain and I use session data. In the beginning, it all works fine until I made a folder in the subdomain. Then on subdomain.maindomain.com/folder/file, the session data won't continue. The session data is to see if you log in, and then in the folder, it is like the data is gone until you go back to subdomain.maindomain.com/file.