You should set the necessary parameters on the plugin settings page and save changes. Captcha is displayed only in case you're using standard pages for registration, login and comments form. If these pages and forms are custom, it is necessary to make some changes to them to make Captcha work.
Follow the next instruction in order to add Captcha to your custom form: http://support.bestwebsoft.com/hc/en-us/articles/202353439
This article related to Captcha by BestWebSoft plugin
Additional resources: Captcha - User Guide
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