This is a common problem especially for fast telescopes with short focal length. There isn't enough "back focus" after the eyepiece holder. You can find the focal plane of your telescope quite easily. Remove the camera or eyepiece and accessories from the focuser, wind the focuser all the way in, then point at the moon. Hold a piece of paper behind the focuser and move it until the moon appears in focus. This is your focal plane. If you measure the distance to the piece of paper from the focuser, that is your "back focus". The sensor is a certain distance behind the front surface of your camera. You can find this information either in the manual or at http://www.sbig.com/support/application-notes-archive/. This has to be less smaller than your back focus distance or you will not reach focus. You also have to account for the thickness of your accessories, such as filter wheel, off-axis guider, and adapters. If the distance is too large, then you may have make some adjustments or compromises. Perhaps you can eliminate the off-axis guider by going to a piggyback guide scope or optimizing your polar alignment and PEC so you don't have to guide. If you have a field flattener, try removing it. Focal reducers tend to greatly reduce back focus. On the other hand, barlow lenses tend to increase it. You should also talk to the manufacturer or distributor of your telescope; they may have some suggestions for you. In some cases they may have accessories or suggestions for simple modifications that may help.