There is a conflict between Fast & Secure Contact Form (SI Contact Form) and Headspace2 - both vital plugins!
When submitting a form, there are issues with the Captcha code/cookie - even though the form goes through, it does not appear to (resulting in a multitude of people resubmitting the form ad nauseum)
The only resolution I've found was to disable HS2 which is a huge pain in the SEO. Other SEA tools do not seem to cause this conflict, but are musch less robust than HS.
I thought it had ironed itself out recently, but apparently not...and no updates to HS2 since June. (SI Contact has been updated a few times).