Probably the most common reason WP fail2ban doesn't work “properly” is out-of-date fail2ban filters.
The "Upgrade notice" in the release notes always says if the filter files need to be updated, but most people don't read the notice and even fewer update the filter files.
Starting with v5.0.0-beta.1, WPf2b tries to check that fail2ban is configured reasonably and that the filter files are current. However, that involves reading files outside of the document root; for "typical" installs that works fine, but if you have a more sophisticated set up (e.g. SELinux, chroot) it won't work.
Assuming it does, WPf2b checks the following;
- Are the filters obsolete?
- Have the filter files been modified?
There will be more filter checks in later Betas.