Hi Vero,
Thanks for replying. Well, my Alfred 3 works well now.
I checked the info on the app and found there's no 'Alfred: ignore' commend. So I reindex the metadata as well as reset the 'search scope'. For resetting the 'search scope', instead of deleting all the default folders, this time I chose to keep the '/applications' folder and then customize it. I don't know which action I took works, but so far my Alfred 3 works well.
Again, thanks for helping.