Alright lemme know if this works. See if this will open up and-yeah. You'll notice that the tip of the bottom jaw is incomplete, it is a hole basically because that whole area was just a complete mess so after trying to go in there and fix it, i just deleted it.

I've never really remodeled a certain portion like that, so i was just experimenting with all the tools. I just extruded all the faces and was planing on merging the vertices to the centre. But i dont understand how that part of the model COULDNT be the problem.