Why didn't my wholesale price apply to an order?

A few things to check:

• The tier price uses the quantity of one item in the cart. For a variable product, 6 red and 6 blue are two separate items, so neither reaches a 10+ tier.
• The best tier reached wins: with 10+ and 50+ tiers, 12 units get the 10+ price, and 9 units get the normal price.
• Check the tiers were saved: the product should be listed on Products → Wholesale Pricing with its tiers.

Also make sure the Wholesale / Tiered Pricing plugin is still active for your store.