I have an app developed in Excel 2007. Some off-site users recently upgraded to Excel 2010 and say when they load the file they can't see the custom ribbon at all. (It appeared fine for them when they ran Excel 2007, before their upgrade.) I've installed Excel 2010 (while keeping Excel 2007 installed) on my PC and the ribbon appears for me in Excel 2010.

Does anyone know if there are any Excel settings the others have that would prevent the custom ribbon from appearing?

I've tried changing seveal settings in the Trust Center but the ribbon still always appears, as it should, for me. I really need some help on this; I'm out of things to try.