SharePoint search results were not correct or would be missing some data. After hours of digging around, I noticed that one Search Scope was empty. To try and fix this, I went as far as reseting the search index, yes, clearing out all the search data. No fun!

Here is what I found to work.

List Settings
Allow items from this list to appear in search results?
Change this to NO
Run a crawl on the Content Source.
Go back to List Settings.
Allow items from this list to appear in search results?
Change this to YES
Run a crawl on the Content Source.

My Search Scope filled up.
Search was working again!

EDIT –
I first wrote this post back in 2013, and Microsoft has made a change in how this works.

1. On the site, select Settings , and then select Site settings. If you don’t see Site settings, select Site information, and then select View all site settings.
2. Under Search, select Search and offline availability.
3. In the Reindex site section, select Reindex site.
4. A warning appears, check the box before Please check this box to acknowledge and agree to the statement above. Select Reindex site again to confirm. The content is reindexed during the next scheduled crawl.

1. On the site, go to the list or library that you want to reindex.
2. In the upper right corner, selet Settings, and then choose Library Settings or List Settings.
3. On the Settings page, under General Settings, choose Advanced settings. If you don’t see the Settings page, you may need to choose More library settings or More List settings.
4. Scroll down to Reindex Document Library or Reindex List, and select the button. A warning appears. Select the button again to confirm. The content is reindexed during the next scheduled crawl.

https://learn.microsoft.com/en-us/sharepoint/crawl-site-content