You can get them to fill in below by pruning them back a bit. That stops upward growth and encourages side branching below. Just snip some of the taller tips back to the next leaf intersection back along the stem. Unfortunately the time to do this is right after it flowers in spring. If you do it now in late summer, you’ll be pruning off the flower buds for next spring’s show. There isn’t enough of the growing season left at this point for them to respond to pruning. I’d wait intil after they flower in spring and do your pruning just as soon as the spent flowers fall to the ground. Also feeding them in spring, every spring, with Holly-Tone will also give them a boost and help them fill in faster.