Off the top of my head, my guess is your dealing with one of two issues.
1. You don’t have ‘multiples’ turned on. In the top right hand corner of the item display window there is an option to select either “Multiples” or “Singles”. You have to switch to multiples for any item category where you plan on using more than one piece.
2. You’re actually adding the items just fine and just not seeing them. This happens to me all the time with facial features. Either they’re too small to notice or, adding them later in work on my piece, the layer they’re automatically inserted into is hidden by items already in the image. Easiest fix imo is, after clicking on the item you want to add and making sure multiples is on; if you don’t see it appear immediately, while still having that item selected, go to the “Transform” tab. First, click on the top-right button of the four buttons under “layers” to bring it to the top. Second, Increase the size to like 300/400% (you can make it smaller again once you’ve found it lol).