Problem with Screen Scraping Region, preveiw works fine, but blank output

I am trying to do screen scraping and setting a region in a pdf document. The preview panel is showing me correct value, but while trying to access the output text it is showing blank value. Any idea what can be possible problem?

Thanks in advance,

Are you using attach window activity?
You sure that it is attaching to the correct window?

Yes attach window is correct. Normal screen scraping is also working fine, but if I mark a region , in that case preview is showing correct result, but output is blank. I have done same process of screen scraping and then mark a region earlier also. For every case if preview shows something then output is same as preview, but this time it is different. Not sure why. Do I need to make resolution better?

If the scrape works fine as a whole then your resolution should be good.
Anything to do with the activity before? Is it loading the pdf page and then scraping the region? Does the script run ahead of the pdf load?