If the condition of the quest is to "Produce items", it means that you will need to produce new items.
If you had already started producing Denim in the Textile Factories before you actually got the quest, that Denim won't count. You will need to collect it and then start producing Denim again, because in this case it's the whole process that counts, not the result.
An opposite example:
Since Planks are actual items (unlike Denim, they are actually stored in your inventory), having them prepared beforehand will work. If you're producing actual collectible items and they don't count toward a quest, make sure that you don't use anywhere else: seethis article for main reasons why players go missing their items.