Filling in with More Stems
Hand tied bridal bouquets have to be kept at a manageable size so the bride can carry the bouquet in one hand if necessary. As you add more flower stems, the shape of the bouquet will begin to form quickly. Turn your vase around occasionally to make sure you are not creating a "one-sided" bouquet. Those are presentation bouquets. This is a hand tie and should look the same from all sides.
Although you'll see more flower heads than shown, I wanted you to see stem placement to see how the grid pattern is getting more dense.
Always add in, filling in the natural "holes" you picture in the blank spaces between the flowers.