Excel Spreadsheet won't save when have dragged cell formulae to other cells

Hi,
I am totally new to C# and .net. I have the task of debugging an application which was written by someone who has long gone. The problem is as follows.

The application opens an excel workbook. If you edit the excel workbook and then save it from the application it works fine in every case I have tried except for one case. It does not work if a cell containing a formula cell is selected and then the bottom right-hand corner is selected and dragged to copy the formula into the cell below. It even works fine if I select the formula cell, do ctrl-C and then ctrl-V in another cell. It is just the dragging thing which seems to cause a problem.

The call is as follows:

currentwb.GetExcelWorkbook().SaveAs(
FileNameWithCxnID,
56,
Missing.Value,
Missing.Value,
Missing.Value,
Missing.Value,
Excel.XlSaveAsAccessMode.xlNoChange,
Missing.Value,
false,
Missing.Value,
Missing.Value,
Missing.Value);

I am totally stumped with this one right now. Can anyone give me any pointers to get me started.

Many thanks to anyone who can give even the tiniest hint!

Tags: C#, .NET, Microsoft Excel