A customer in my VIP group is not getting their group discount at checkout. Why?

Check these:

1. Signed in: group prices only apply when the customer is logged in to their account at checkout. A guest checkout with the same email gets normal prices.
2. Assigned: open them under Customers → Manage Customers and check Customer Group on the Profile tab.
3. Group active: on Customers → Customer Groups, the group's Status must be Active.
4. Rules: the group needs at least one discount rule. A rule with a Min cart only applies above that amount, and a Specific categories rule only to those products.
5. Plugin: the Customer Groups plugin must still be active.