When you add or save a field in UsersWP > Form Builder, the browser sends the field settings to wp-admin/admin-ajax.php. If that request comes back with a 404, the field is not saved and nothing happens on screen. A 404 on admin-ajax.php means the request never reached WordPress, so the cause is on the server or in a security tool.

Confirm the error

1. Open the browser developer tools (F12) and go to the Network tab.
2. Add a field in the form builder.
3. Click the admin-ajax.php request and note the status code and the response.

Common causes

• A web application firewall or mod_security rule. The form builder sends the field settings in the URL of the request, and long URLs with HTML in them can trip a firewall rule. Some hosts return a 404 or 403 page when that happens. Ask your host to check the firewall log for the blocked request.
• A security plugin that hides or renames wp-admin. Make sure admin-ajax.php is still reachable for logged in admins, or turn that feature off to test.
• Server rules in .htaccess or the Nginx config that block requests to wp-admin. This is more likely on staging servers that sit behind password protection or IP rules.
• A broken or partial plugin upload. Delete UsersWP from Plugins and install it again. Your settings and data are kept when you delete the plugin, unless Remove Data on Uninstall? is ticked on UsersWP > Uninstall.

If the request returns 200 but the field still does not appear, check the response text. A message such as “Column creation failed” points to the database instead, see Form builder error: “Column change failed, you may have too many columns”.