Hi Frederick -
250 MB isn't that big, so I'd guess size isn't an issue here.You mentioned that you eyeballed the original rows in Excel and saw they had values. Are you connecting directly to the same Excel doc with Tableau or moving your 250 MB of rows out of Excel to a different data source and then connecting to that "different" data source with Tableau?
What happens when you take some of those rows you identified as "problems" and drop them into a new worksheet, then connect to that? What happens if you export these rows as a CSV and connect to the CSV?