It sounds like you are having a domain switching problem. usually this happens when going between using the www in the front of the address and not, for example:
mydomain.com
and
www.mydomain
As far as the Session Cookie is considered, these are two separate domains, so seperate browser sessions
check the RootURL in the wa_settings table of the powerstore database, and make sure to access the site to add items to the cart using the same address, either with or without the www, depending on how it is set in the database.