Just a few ideas: Always use new brass. Lube the inside of the case mouth before expanding. Expand in small increments or start with brass as close to the finished product as possible (35 whelen, 9.3x62).

If you continue to have trouble, I have had good luck fireforming with no bullet. Put about 12 grains of bullseye or other fast pistol powder in the case, put a little piece of tissue over the powder, fill remainder of case with cornmeal, top with bullet lube, crayon or wax to hold it together, NO BULLET, point gun straight up, fire. Trim to length.