Wheels-On-Air Plans Third Model Trouble

So Wheels-On-Air's third model is performed (reached about 7.6 meters), but the problems lies with reaching the 12 meters. We do not know how to modify it so that it reaches the 12 meters. We are considering shrinking down the torso (egg carton) but our balloon won't be stable this way. It goes relatively slow, but steady and straight unlike our previous models. Further updates regarding our opinion on race day.