Make sure the data types are consistent: right-click the date fields in the left Data Pane > Change Data Type > Date (or Date & Time)
If this doesn't work, perhaps you could post a subset / sample of the data in a packaged, twbx workbook?
Hi Jim. Thanks for your reply. Yes the data types are both the same (Date). Its very tricky for me to post a subset of the data due to the commercial nature. Perhaps i will try to "re-create" the problem using dummy data, and use that to post.