Cannot login on subdomain

I recently installed a network using sub-domains. I can login through my primary site which is

http://mmgtv.org

but when I try to click on dashboard for my secondary site which is

http://sentient.mmgtv.org

or if I try to go to http://sentient.mmgtv.org/wp-admin/

I cannot log in!!! This is so annoying. Can somebody please help me. I’ve been researching all over the Internet for this and I can’t find a solution.