All new products need to have a "REGULAR PRICE" tag added so that it appears in this collection. When an item goes on sale the REGULAR PRICE tag must be removed and the SALE tag added. When the item goes off sale, then the SALE tag is removed and the "REGULAR PRICE" tag is added.
This is necessary so customers do not receive an additional discount on Sale items when using a coupon code.
When creating coupon codes, make sure to select "Applies To" "Specific Collections" and select the "REGULAR PPRICE" collection.
Here is a video about how to manage this: