Microsoft Excel: When coworker enters data, formula does not update?

I'd check the excel file size to make sure it hasn't ballooned up...I've had instances where a workbook get corrupted and have a 40k workbook end up being over 1mb. If it is a corrupted workbook, the best option would be to recreate the workbook. You can also try protecting the worksheet to prevent your coworker from making changes to your formulas.