Hi, 

How often are you checking for water? They may be drying out faster due to the slope or the immature root structure. Did you loosen the roots when planted?

Since these were planted this spring and don't have a root system that is as established as the other deutzia, they could have gotten too water logged in the spring with our heavy rains and that could have caused some root rot and loss and now they are struggling with the high heat and drought. 

They are pretty resilient though, so you can try to cut them back to the green growth underneath that looks healthy and less sparse. They may just need to get established and within the next year or 2 they should be filling out nicely like the others. 

If you cut them back now and keep up with watering through our drought and heat, and they continue to keep dying back and looking sparse feel free to send additional photos. 

Emily

Thanks for your answer