Answer: Now that spring has arrived, renovate your spireas by pruning out all the obviously dead branches and then shortening some of the healthy branches. This will result in a flush of healthy new growth which should cover any bare spots by mid-summer.
As for avoiding the problem in the first place, what is the watering situation? Most plant problems are caused by either underwatering (water doesn't penetrate deeply through the entire root system) or overwatering (roots need oxygen to thrive and overly wet soil doesn't allow that).
Best wishes with your spireas!
Q&A Library Searching Tips