desktop search: copernic vs. google

i installed copernic on my computer. it turned out to be better than google. google and yahoo both failed to find one text file that i have although i installed google all text search plugin. i don't know why. google can find other files of that type but not the ones in a specific directory. on the other hand, copernic worked flawlessly. it found the file as soon as i added the file type to additional text file list. also the preview with highlight feature is very useful.