Global Shopping Cart not working now Domain Mapping is installed

Ever since I've installed the domain mapping plugin on a network of sites I'm building for a client I've had nothing but issues!

MarketPress, which has Global Shopping Cart enabled now refuses to actually have a Global Shopping Cart.

Until I mapped domains to the network, this worked like a dream... Now if you visit and add the product to your cart, then go to another site in their network such this page and add a cd to your cart

The original product you had in your cart has vanished! I take it what I've promised my client based on what your plugins say they can do isn't actually possible then?

All your plugins say multi-site compatible, domain mapping compatible, buddypress compatible, so am I missing something in my configuration somewhere?

Any ideas on how I can make global shopping cart function again?

Many thanks in advance.