You may not have enough back focus with the focal reducer in place.
A focal reducer is an accessory that screws onto the threads on the back of your SCT’s optical tube assembly (OTA) to give a smaller and brighter image at prime focus. It yields lower magnifications, a wider field of view and a faster photographic f-stop than the original f/10 configuration of the OTA. To do this, it shortens the converging cone of light from the main optics, bringing the focal plane closer to the back of the telescope.
Focal reducers (also called telecompressors) are described in terms of the magnification factor. Celestron makes a focal reducer that has a magnification factor of 0.63. The resulting f-number with the reducer on the scope for a SCT that has f/10 optics is this factor times the f-number - f/6.3. This reducer is often called the f/6.3 focal reducer.
The distance from the back of the telescope to the focal plane is commonly called back focus distance or simply back focus. Typical values for SCTs are in the range of 5 to 6 inches. This is usually plenty of room to focus things like cameras, 2” visual accessories, etc. attached to the scope.
The back focus is significantly shortened by the more highly converging beam of light produced by a focal reducer. For the Celestron f/6.3 reducer, the back focus on an 8” SCT of 5 inches is shortened to only 3 inches. Depending on your accessory, you may not have enough back focus to focus your image with the focal reducer in place.
With f/3.3 reducers from other manufacturers, the situation is even more extreme. An 8” SCT with an original back focus of 5 inches is shortened to a back focus of just 1.7 inches. This is shorter than the distance from the lens mount to the film or chip plane for many SLR-type cameras. In this situation, you cannot focus your camera using the focal reducer on an SCT telescope.