There is a setting to Disable the captcha for logged in users, make sure you have not enabled. Also it could be a problem caused by another plug-in that conflicts with this one and bypasses the captcha validation function. Always test the captcha when you install new plug-ins or update plug-ins just to make sure it still works properly
Are you not using the contact form of Mike?
Because SI CAPTCHA is a stand alone version whereas FS contact form comes with a integrated secure Captcha.
We use Mike’s form and captcha since years now – we are also extremely visible on the net and have no spam coming in via our forms, only manual entered entries reach us.