Joomla Virtuemart "Add New Shipping Address" problem

I have a problem with my Joomla 1.5.22 and Virtuemart 1.1.8 site in that when a user tries to add a new shipping address to their account they are unable to. You fill in the form and upon saving it diverts to the shop index page and the address hasn't saved. This happens on the checkout page too when you try to add a new shipping address. Only the default billing is shown as a shipping address. I've trawled the VM & Joomla forums and there's much mention of DB tables and other things I dont understand. Its a startup company and a self created site. I need this fixed urgently.