Cannot run the macro 'XXX'
 

I am developing an Excel 2007 add-in compromising of an Excel Add-In (.xla
file) and a COM Add-In (managed code written in C# in Visual Studio Tools For
Office). In the start-up function of the COM Add-In I need to call a macro
on the .xla file.

In the installer application for the add-in I am changing the registry keys
in Excel Security so that:
- The Trust access to VBA project object model check box is ticked;
- All macros are enabled.
On a large proportion of computers that I test the add-in on, everything
works fine. However there are a few computers which (when attempting to run
the macro) fails with the following error:

"Cannot run the macro 'XXXXX'. The macro may not be available in this
workbook or all macros may be disabled."

I assume that there must be some further configuation in Excel 2007 to allow
the macro to be run. Any help in pointing me in the right direction would be
greatly appreciated.