I'm really surprised that with that much mass, the soap didn't gel all the way. If I'm not mistaken, those are the same molds that Ariane Arsenault uses in many of her videos. I would try emailing her, to see if she gets a full gel. She did have a baby recently, and hasn't been posting videos, so she might not respond right away, but it's worth a shot. What I do, with some of my small, individual silicone molds that are too small to gel on their own, is to put them inside a small styrofoam chest. This holds any heat they generate inside the box, and they always gel with no problem. So I might try something like this...
http://www.homedepot.com/p/R-Tech-3...89-Foam-Insulating-Sheathing-310873/202530470, and cut it to make a box, with a top and four sides that would slip over your mold to hold in the heat, taping it together with duct tape. The only problem is, if it's generating enough heat for the center to volcano.....it's hard to predict what might happen. But maybe the styrofoam would hold in enough heat for the soap to heat up more evenly, and possibly avoid the volcano situation. I don't know, but it's a nice mold, I wouldn't give up on it yet. Good luck!