Sharepoint search does not return all the documents I expect

Hi,

I created a site column (single line of text) for article codes and added it to my content type. Then I valorised the column for all documents (500 more or less) with their article codes, re-indexed the site and waited a couple of days.
I give an example of an article code, some of them are formatted like this: "ABCDEF 12345.01/A + Z99.123" where the initial part (ABCDEF 12345.01/A) is present in multiple article codes and only the final part (Z99.123) varies
Now the problem I am experiencing is that if I search for "ABCDEF 12345.01/A" I only find a few (4) of the documents that have this string in the article code, while using the filter on the column I can see there are many more (12).
Would anyone be able to help me understand if I have misconfigured the column or if there is a more serious problem
Thank you