Problem
You're using Client Specific Pricing (CSP) and the member discount is not applying on the invoice when printed, even though the member price is blank in the Pricebook and the customer has an active membership on the invoice. With CSP, the system marks up the standard price set up in the CSP rule and sets that same standard price as the member price.
Solution
To apply the discounts, set a flat rate for the items in the Pricebook you want to discount for members rather than setting a CSP.
Alternatively, you can remove the rate sheet from the invoice. If the rate sheet is removed from the invoice, the member discount will apply to the items.