#10 Shopping Cart Error for repeat purchase

[Anonymous]

Using CRE Loaded 6.1a with patch 001.

I've been testing purchase with the Paypal Sandbox.

When a customer first logs in, adds to cart, checks out,
it all works fine. When he returns to the commerce
store he gets a message to continue, no problem here
until he decides he wants to purchase another item.
When they try to add to the cart, the shopping cart
remains at zero and does not except any additional
purchases.

How can I fix this?
Thank you,
Jim B.