Sometimes, though not often, your X1 may display incomplete search results, or an out-of-date index.
The easiest way to address this problem is to manually begin an indexing cycle for the application you feel is producing incomplete search results.
1) Go to Tools>Options>Email>Thunderbird (assuming I have missing data from this application)
2) Select "Index Thunderbird Now" to manually begin an indexing cycle. This function exists for all X1 connectors within the Options Menu (Index Outlook Now, Index Files Now, etc.).
While my index is currently up to date, I notice incomplete results only a couple days later!
Please Note: If you do not choose an indexing schedule for your connected applications--like the above selection of "Never", you won't receive regular updates to your index data. Additionally, if you want to have regular updates, make sure you use the "Every" selection and determine at what points of the day you need to most complete index and search data.