An org admin can check the following if a discount is not applying when it should:
Go to Settings > Pricing > Discounts
Check to see if the discount has been added to the pricing model. The "In Use By Models" Column should have a pricing model name.
If the column says "Not In Use" click Settings > Pricing > Pricing Models.
Select the pricing model you want to add the discount to, and then add the discount in the discounts box and click save.
Return to the proposal and click update pricing.