There are three main reasons for this problem. 1.) Over-proofing the dough causes the dough to over-inflate and eventually collapse. This can causes gaps and holes in the bread. 2.) Uneven filling distribution can affect the baked bread. Areas with excess filling will be dense while areas without enough filling may bubble with gaps 3.) Improper rolling is the most common problem. Roll the dough tightly and evenly to prevent air pockets.