A wholesale customer was charged retail

This is the most reported issue and it is almost always one of the five causes below, in this order of likelihood.

1. The buyer was not signed in. Shopify only tells the discount function who the customer is on an authenticated session. If they checked out as a guest, there is no customer, no tag, and therefore no discount. Confusingly, Shopify attaches the order to their profile afterwards by email, so the finished order still shows their name and history and looks identical to a signed-in one. Ask the buyer to log in before adding to cart.
2. The customer is not tagged, or the tag matches no group. Open the customer in Shopify and check the tag exactly matches the group's tag, including hyphens. A tag with a typo, or a per-site tag you use for something else, matches nothing.
3. The group is inactive. An inactive group is ignored entirely.
4. You are on the free plan and hit the monthly cap. The free plan covers 10 wholesale orders per calendar month. Past that, discounting pauses until the month rolls over and buyers pay retail. Your dashboard shows the counter.
5. The order was not a normal cart checkout. Draft orders and orders created through the API do not run the checkout discount function. Use the app's own quote and purchase order flows for those, since they apply wholesale pricing when the draft order is built.