As long as this thread has some legs. Here are the problems I have run into over the years.
Light leaks are pretty uncommon with Grafmatics. One thing to watch out for is the felt pad at the end of the chamber can come loose. I had one that folded over. When I figured out the problem it was a quick fix but there was some head scratching before then. The light leak will show up in the shape of the cutout of the septi.
The septi can bend so look at them before you load film You want them to be as straight as possible. I use my fingers if there is a problem. I wouldn't use pliers as mentioned above. You don't want to mark them and cause scratches/wear on the mechanisms.
Make sure the film fits snugly in the septum. I have had film come loose before and it jams the mechanism. I adjust them so that the film is just difficult to load. Also, check that the film goes under the end of the septum. If you have strong hands this can all be done with your hands and not tools. There are two kinds of septi that I have encountered. One has the folded over edge go the full length of the holder and the other has a kind of friction pressing and the far end. Neither is better than the other in my experience. They both should be adjusted appropriately.
Don't forget to push the dark slide back in. The septum rests against the darkslide to position it properly for the exposure. I had a friend for whom I had built a Polaroid conversion complain to me about unsharp images with the Grafmatic. It turned out he was leaving the darkslide open since that is what he was used to doing to avoid light leaks while shooting with regular holders.
Every so often I lube the channels in my Grafmatics with wax. It keeps them running smoothly. The first time I did it made a big difference. You could probably use other lubes as well. Obviously you would want something that is dry.
If you ever get to a point when you are out shooting and the Grafmatic jams or the darkslide is met with a lot of resistance, tilt the camera back and try to jiggle it a little to let gravity do its work. Never force anything with a Grafmatic. I have only had one jam in the years I have been using them and it turned out to be a loose sheet of film as I mentioned above.