If Calibre is omitting books from a search, then:
1) there are subtle differences (like an accented character) in the searched fields.
2) there are some not so subtle

differences. 'The Series Name' vs 'Series name' (

just had one of those ), One version is Plural Mystery vs Mysteries
You have a VL in effect? (search only look at the current view. Be sure to switch back to All Books)