Check what "Print" is set to in Excel
In Excel, go to File > Print. The settings panel shows what will be printed. By default, Excel prints only the active sheet (the one you've selected). If you have multiple worksheets and want all of them in the PDF, you need to change this setting. Click the "No sheets selected" dropdown (or similar) and choose "Entire workbook". This tells Excel to print all worksheets, each as its own section.