Empty Cart on Checkout

Hi…

A few of my clients complain that although they checkout and pay for the items in their cart, the next time they visit the website all the items they have previously ordered is again in their cart.

I have tested it out a couple of times myself, but didn’t see that happening, but at least 12 of my customers complain about it.

Any ideas?

Thanks

http://wordpress.org/extend/plugins/woocommerce/