Question regarding VerticalLayout - list is not populating correctly except during Start()!

Hello! I am currently working on a project to make a dynamic inventory screen. The inventory needs to populate a list of items at runtime based on clicking a tab for that type of item (weapons, armor, etc.) I am using a ScrollView with a vertical layout to fit the items as described in the tutorial here: https://unity3d.com/learn/tutorials/modules/beginner/live-training-archive/creating-scroll-lists-at-run-time

I followed it and everything worked great! I use the following function to populate the list

 void PopulateList(List<Item> items)
     {
         foreach(Item item in items)
         {
             // Create the list item in gamespace
             GameObject list_item = Instantiate(inventory_list_item) as GameObject;
             list_item.transform.SetParent(contentPanel.transform, false); // child it to the panel object that we will populate the ScrollView with.
             List_Item itemScript = list_item.GetComponent<List_Item>();
             // Send it a reference to the item that it represents
             itemScript.BuildListItem(item);
 
             // We want the item to register click events.
             list_item.GetComponent<Button>().onClick.AddListener(delegate { InvItemIsClicked(itemScript); });
         }
         inventoryUpdated = false;
     }

So the problem happens as follows. When I use Start() to call this method and pass it a list, it works fine:

However, if I call this function by clicking a button, it sticks all of the elements at one spot. For example, after clicking the junk items button:

I am not sure exactly what is going wrong here, since the list is the same and the method being called is the same. Is a VerticalLayoutGroup only set up during Start(), or is there something else I need to do?

Thanks for taking a look at this!