Can't open Excel files

I have a C# application (2005 .Net) which will loop thru files in a directory
and read Excel files (currently using Excel 11). There are usually multiple
Excel files in the directory. Up until a few weeks ago, it was working fine.
Now, when the the Open method is called, most of the time it returns a Null
value. If I stop the application and start it back up, it is usually able to
open up the file it had failed on before. I added in a try..catch routine to
see if an exception was thrown, but none was. Where else can I look for what
is causing the problem? Below is the Open command I am using.

Workbook workbook = application.Workbooks.Open( filePath,
false,
true,
Missing.Value,
Missing.Value,
Missing.Value,
Missing.Value,
Missing.Value,
Missing.Value,
Missing.Value,
Missing.Value,
Missing.Value,
false,
Missing.Value,
Missing.Value);

Thanks for any help.