Have you made sure your annealing schedule is correct? Maybe they are cooling too fast in the kiln. I anneal at 968-973 & after the last bead goes in, I give it an additional 30 minutes soak time (I usually make about 12mm beads.) I ramp down to 800 at 150 degrees an hour and hold for 45 minutes, then ramp down at 350 degrees per hour to 350then shut off. This is probably ultra conservative, but it works for me. I think cooling too fast is the main reason beads crack.