The problem may occur if you have installed and activated some caching plugin. If you have one, just try to deactivate it and check the Captcha plugin.
If you have no access to your Dashboard, it is necessary to deactivate the plugin manually. Please complete the following steps:
1) Go to the your_wp_root/wp-content/plugins folder via FTP;
2) Find "captcha-pro" folder and rename into "captcha-pro1";
3) After you log into your dashboard, you can rename the folder back.
This article related to Captcha by BestWebSoft plugin
Additional resources: Captcha - User Guide
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