what is the full address in the address bar when you first have items in the cart?
What is the full address in the address bar when the cart is empty?
Please copy and past these two addresses from the address bar for comparison.
My guess is that the domain is changing
this can happen if you access your site at:
but paypal redirects you back to
these are not treated as the same domain by the session cookie, so a new session will be started.