I used the Find Free Full Text function but did not get all the free texts

The Find Free Full Text feature uses PubMed's API to search and retrieve full text articles from the PubMed Central (PMC) Library. 

On the PubMed website, you will notice links to Free PMC articles as well as links to publishers websites. The API does not retrieve articles from these publishers. Although they are free, there are copyright issues that exclude these from the API (PMC database) as they're currently not classified as Open Access. It's a licensing issue that restricts redistribution. These articles are still free but need to be retrieved manually.

The PMC Open Access subset contains articles that are still protected by copyright, but are made available under a Creative Commons or similar license that generally allows more liberal redistribution and reuse than a traditional copyrighted work.  

The majority of the articles in PMC are subject to traditional copyright restrictions. They are free to access, but they are not Open Access articles in the specialized sense of that term.