Different items have different rarity attributes. The higher an item's rarity, the lower the chance that it will appear at the merchant. When you are not seeing a certain item appear at the merchant even after many refreshes, unfortunately, it might just be that you were unlucky for that period.
However, you can increase your chances of finding a particular item by purchasing more of the equipment from that merchant. This will allow for more slots to be available for the item that you are looking for to show up.
Additionally, as you improve your relationship with the different bonds in Postknight, you might receive certain useful items as a gift from them sometimes!