There are a few factors that may contribute to your problem. It may be because not enough heat is applied (medium to high heat but I normally use a high medium), or it may be because when you iron them, you're going across the surface so fast that its not getting equal amounts of that heat. You'd want to go across the surface slower than you normally go with ironing an article of clothing. The best way to may sure that all the beads stick together is by letting it cool on the board after ironing it while leaving the wax paper on it. The parts that stick to the wax paper are the ones fused together, the ones on the board aren't melted. Simply go over the area again and it should help piece it together. Yeah, big pieces are awesome to make but they're a little frustrating. Aside that, I really hope that you found my advice useful and will help you in your big pieces! Good luck!