Can't send email. Form data is not in header

This is the first time I setup a contact form in Grav. I use the sample form in https://learn.getgrav.org/forms/forms.

When I submit my form, I am taken to homepage, no email is sent, no errors..

As I've debugged in Form plugin, $header->form (https://github.com/getgrav/grav-plugin-form/blob/master/form.php#L49) doesn't exist so onFormProcessed is not triggered and Email plugin is not called. Form content is still in $_POST, but not in Grav's header.

I really appreciate any suggestions from you to help me find out what causes this problem and fix it.

Problem solved. Don't know when I added "action: /" to the form. Arrggg!