if an Excel file is older than Excel 97, JMP will try to use some much older code to open an Excel 5.0 file. That is only available through Open All Sheets. However, files earlier than 5.0 often fail to open at all. The simplest solution is to open the file in Excel and use File->Save As to specify either Excel Workbook (XLSX) or Excel 97-2003 (*.xls). Then save out the file using a new name or overwrite the existing file. JMP can now open the file.
If you have a large quantity of these older files, then it might be worth writing a small about of VBA script within Excel to batch process them.
If you are sure the file is a newer file, that has perhaps been generated by a product other than Excel, it might not have been properly generated.
I hope this helps,
Brian Corcoran
JMP Development