Dropdown option does not work

Normally, this is because in the YITH Plugins section> Ajax Product Filter> Frontend the options for the title widget and wrapper widget have a wrong settings.

To properly configure, see this article : 

https://support.yithemes.com/hc/en-us/articles/219043698-Ajax-Product-Filter-The-widgets-do-not-update-after-a-filter-is-applied-Do-not-you-see-RESET-FILTER-The-filters-do-not-pass-the-state-active