Alfred uses an intelligent search scope to be fast and efficient. You can expand your search by prefixing your search with the keyword "open" (or by pressing the spacebar).
If you can't find the files you need with the "open" keyword, follow these troubleshooting steps:
1. Can Spotlight find what you're looking for?
If Spotlight cannot find the files either, go to Features > File Search > Advanced tab in Preferences and click "Rebuild index". This is due to the fact that Alfred and Spotlight both use the metadata gathered by OS X to find results so if Spotlight can't find your file, it's likely that it isn't correctly indexed.
Once this completes, type "reload" into Alfred to refresh his cache of your Applications.
Note: Be aware that reindexing can take up to an hour to complete, during which time Alfred and Spotlight's results may be incomplete. Time for a cup of tea, perhaps?
If you notice that Alfred appears to find an application that Spotlight doesn't see, this can be because Alfred has previously cached the application. Typing "reload" into Alfred will refresh the cache so the app will disappear. You'll then need to perform the indexing step above.
If Spotlight does find your file, move on to the next step.
2. Are your Spotlight Search Results checked?
Launch your System Preferences to the Spotlight preference pane to ensure all checkboxes for the file types you want to search are selected. Spotlight allows you to choose which file types are indexed by OS X. As Alfred relies on the same metadata, it's essential that the boxes are checked for all file types you want Alfred to find.
3. Is your search scope set correctly?
It is possible that you're searching for something that is outside of Alfred's default search scope. To review your settings, go to Preferences > Features > Default Results. Check that you have ticked to show the file type that you were searching for or add specific paths to your search scope. This will tell Alfred where to look.
Don't forget that if you're searching for files, you can also use the keyword "open" (or press the spacebar) prior to typing your query. You can also adjust the search scope specifically for file search (under Preferences > Features > File Search).
If your search scope is correct, move on to the next step.
4. Is your application cache up to date?
If you are looking for an Application, it's possible that your application cache has somehow become outdated. Type the word "reload" in your Alfred window to refresh your app cache.
You're still here? Check the next step.
5. Are you searching for the correct file name?
Ensure that the name of the file you're searching for is accurate, as you may be searching for a file name that doesn't exist. Consider using the * (star) character as a wildcard if you only know part of the word you're searching.