Are the data sources being joined or are you using data blending? If you have joined them, using Data Blending may help resolve this. See: http://community.tableau.com/thread/135607
If this does not help, you may need to post a packaged workbook that demonstrates the issue so we can better understand what is happening.
thanks for your reply. I am using data blending.
I will post the packaged workbook shortly, thanks.