Hi João Vasconcelos

While it is possible there is a problem with your shortcut key of F3, I doubt it very plausible at this stage. 

What I understand is that you have very high reason to believe a word should belong in your TM, but it is not showing up when running a concordance search.

There are a few area's we can explore starting with:

Search Validity - does the word you are looking for exist in your TM?
This can be assessed by:

1. Open your TM within the Translation Memory Maintenance View 
2. Perform a search for the word that you are trying to run a concordance on, when in the editor view

Please see my example where I search photo as the source. I have about 5 results. Therefore I would expect some hit when performing a concordance search on photo when in the editor view
*Take note, you can also search on target side as noted in step 3

Assuming you can find your word, we can now explore other areas that affect the concordance search.

Project Settings

1. Go to your project settings - Language Pairs - All Language Pairs - Translation Memory & Automation
2. Ensure your TM is listed but also its enabled for concordance, as seen in the screen print below

Assuming yes, then we can review the Search Settings by going to search and checking to ensure your Concordance settings aren't unrealistic. Mine below are the default settings for most users
 

Assuming that is all still ok, go back to the editor view. Lets check the Editor UI and ensure you are looking in the right area for the search results.
So try run another concordance search.  You could avoid the shortcut (just to test) and use the context menu instead.

At times the view is not automatically changed from familiar TM results pane, to the concordance search results - so please do confirm

Looking forward to your confirmation before we explore other areas

Lyds