I'm often surprised (disappointed) to find that when selecting an English word in a resource to get a definition, that there won't be any hits in my main English dictionaries (Concise Oxford and Merriam-Webster's). I've learned it's usually because the word selected for lookup is not in the exact form as the main entry in the dictionaries. So for example, selecting the word "look" will result in a hit, but "looks", "looked", and "looking" will not.
I don't see any "settings" option in Logos that deals with this but am hoping there's something somewhere that I'm just not aware of.