Hi ClayDevin and welcome!
While I can't be certain, I am going to guess that this is an issue with your xpath. To create a flag that is row-specific, you'll need to use the current() function in xpath to make sure it loads the Due Date from the current row and uses that to compare to the current date which can be retrieved via the Today() function or Now() function depending on whether or not your Due Date is a date or datetime type.
Bottom line: this is pretty easy to due. Let us know if you can't figure it out and we'll attach a sample. Don't forget to search the site (see line at the top of this page, underneath the header) for Current() to find pages referencing the xpath techniques. Greg Collins has a few blogs on this.
Best of luck!