_ExcelWriteCell() Doesn't Write Text To Cell

I have a script that opens an Excel file that is stored on a network drive and updates some of the cells using _ExcelWriteCell.
Occasionally, the Excel file will open just fine but the target cells don't get updated.

What could be causing this behaviour?

The relevant lines of code are as follows:

$oExcel = _ExcelBookOpen($FileName)
_ExcelWriteCell($oExcel,$FileNumber,"E" & $k)