Have you actually loaded the cargo into the aircraft?
Open the Animation Control Panel (SHIFT+3), the click on Loader boxes, and when the Load Managers open, click on Show Loader.
To load ULD's into the 777F, you need to open the main cargo door, and the forward lower cargo door, then use the loaders to take the ULD's to the 777F, and load them into it. Use the mouse to move the ULD's from the doorway further into the aircraft. Each time you add a cargo ULD, the weight of it will be added to the 777F's total weight.
This is explained in the ‘777 Captain’ FLIGHT MANUAL Part I – User’s Manual (csx777_manual1.pdf), on pages 39 and 40. You can download it from here: https://captainsim.com/products/x777/manuals.html