I am currently using the built-in inventory GUI in my game. However, the scroll up and down button won't appear. Whenever I get more than 12 items in my inventory, the items after the 12th are not visible, and the buttons to scroll up and down do not appear on the GUI.
Try putting this line:
game.num_inv_displayed = 12;
before the line "InventoryScreen();" in the global script function show_inventory_window.
How are you adding objects? If it's not with AddInventory(), you might need to call UpdateInventory() after you do it.
I tried that but it didn't work. I am using AddInventory and LoseInventory too. Actually I just realized that the inventory window and inventory GUI are 2 separate things. I think I just want to use the standard window for now.