It could be many things, I suspect that the file is indexed, as the other phrases do yield the result. I suspect that it might be matched, but somehow not displayed correctly on the results page, as I am always seeing the same number of results there.
Can you please add temporary back-end details? I would like to check the configuration and do some debugging.
The file was actually matched, but there are many other more relevant matches preceeding it. I have increased the limit to 150. When searching for “Entro il 15 dicembre” and then if you go to page 7, the result is there: https://i.imgur.com/CsqyC8c.png
The pagination on your theme is on the bottom of the results: https://i.imgur.com/wW8Wjq6.png