It's quite strange because I got aware of this after a few weeks the site is online, but it looks like group price rules are not working.
If I create one (e.g. product X must have €25 price instead of €29 for group G) I can see the correct price on the catalog: when a user in group G is logged, he sees price at €25, but as he adds the item to the cart, it gets loaded at €29, and so it remains until the end of the process.
What may be wrong?