It might have to do with the NBT tags. This is extra data about an item. There is a chance the item you used to set a tradeshop sign is not the exact same as what is inside the chest.

An example, I think that a banner that has been placed does not stack with banners that have never been placed.

An example with enchantments, I don't think the same tradeshop sign can be used to sell two different swords if they have different enchantments on them.

An example with copies of books, I lost the copy of a book I was using to make copies of a copy. So I made a new copy of the same original book, but the copies of that second copy do not stack with copies from the very first copy.

Also, written books that were sold through a tradeshop and returned back to me do not stack with books that have never been sold.

This applies on /skyblock. I think the same goes for /economy

A solution is to remove the old tradeshop sign and create a new one using an item you couldn't sell, but was in the same chest. And keep remaking it if you find later on that there more of the same, but not exactly the same items remaining in the chest.

I haven't set up tradeshop signs in a long time though. So I hope another player can help support or refute me.