First thing: Have you removed "Both" the lens cap (clip in) and the lens ring (slide-on)? If not, theres your issue. If the lens ring (slide-on) is not removed you are limited the FOV to around 140 degrees.
The Sigma 8mm Fisheye is a 180 degree lens, BUT! the camera you are using is not a full size sensor camera it is what as known as a APS-C sensor. These sensors are smaller than a film camera focal plane of 36x24mm and this is what the Sigma 8mm was designed for and bases it rating of 180 degrees on.
Don't worry though. Using the Sigma 8mm on a APS-C small sensor DSLR with 1.5 or 1.6x multiplying factor can actually be better. Because it using a lot more of the available pixels. You will need to shoot 4 images in rotation with the camera on a panohead in portrait mode/positioning (sideways). Which it appears you have done.
What you can do to help close the top/zenith hole is to tilt the camera/lens up around 4 degrees. This will close the upper zenith hole completely but open the lower/nadir hole slightly. Though this normally only effects the tripod footprint anyway and you can cover this with a tripod cap/logo.