Hi Chris,
I had a similar problem to you and found a solution eventually. The best way to test for problems is to watch the task run while you're logged in as the same user. Using this method I was able to find out that 2 things in particular weren't helping
Macro Security - I've had to set this to 'low' (in Excel under Tools, Macros, Security) so that the 'Enable Macros/Disable Macros' prompt doesn't appear when Excel opens a macro workbook. This is far from ideal, so I'm now looking into digitally signing the macros.
AutoRecover - This has had to be completely shut down (in Excel under Tools, Options), as it was halting macro execution by forcing the 'user' to make a decision on what to do with potentially recovered workbooks from another session. There appear to be two settings for this - one referring to a specific workbook, and the other to do with the actual installation of Excel for that particular user - that's how I've interpreted it anyway. Both are in Tools, Options.
Unfortunately since last week the same problem has returned where Excel appears to open and then do nothing. I've stripped a test right back down to a macro which literally quits Excel as soon as it runs, and even that isn't currently working. I suspect a recent Windows update and believe the problem is either user permissions or something new in Excel - unfortunately that doesn't narrow it down very much.
Good luck,
Peter.