Here is a list of potential solutions to try if you are unable open an Excel sheet in full-screen mode:

1. Open the file in Full Screen mode by navigating to the View tab and selecting Full Screen from the window panel.
2. Restart your computer to see if it helps the Excel file open in Full Screen mode.
3. Disable the worksheet’s zoom and ensure it is at 100%. You can do so by going to File > Options > Advanced. Locate the Display section and set the value of Zoom to 100%. Don't forget to click OK.
4. If you have multiple documents open within Excel, create a new workbook and copy the data you need across.
5. Try opening the file in another version of Excel, which may be compatible with the file’s version.