I have a new working AbanteCart installation that was working fine. However, something has happened recently, and now on some pages the cart block in the top right corner doesn't reflect the cart contents - it shows it as empty / 0. If I view the actual cart page there are items in there, and on _some_ pages the cart block is working correctly (eg; the About page). But on other pages the cart block always shows zero. Any idea what might have happened?

Example: Cart page shows there are items in cart, but cart block on top right of main page shows cart is empty!