If your email field is not set to required, you cannot assign it to the reply to email setting in the form builder settings. As such, the form builder does not know which field to choose for the email reply to field, since it is optional and not required, the reply to function will not work because it is variable. You need to set the email field to required and then set that in the settings of your form to that field, then it will work.
Thanks,
Joseph
VFBPro Support