I'm sorry but your post doesn't make it very clear what the problem is. Are the plants dying because they are in too much shade? Too much water? Not enough water? Poor soil conditions? Overcrowding and/or neglect? Without knowing what the cause is behind the decline, it's a little difficult to make any meaningful suggestions.

I would also suggest that you may be seeing the signs of some cold damage as well. A significant portion of the eastern half of this country just got through the second rather severe winter in a row with far below normal temperatures for more than a week at a time. That can knock a lot of typically hardy plants back with a wallop.

If you can provide greater detail and even some photos to help illustrate what the issues are I'm sure we can come up with some appropriate suggestions.

All of the above. They were planted 25 years ago on a hill with water runoff. There is a sprinkler system on this hill, so they've done okay until now. Half are in sun, half in shade. Yes, the soil must no doubt be poor. They've been fertilized, but the soil is original. They are now overcrowded because they've overgrown. Consequently, the bottoms are dead and the tops of the shrubs are reaching for light.

Yes to the cold damage as well. I didn't think it was as bad a winter as it obviously was, but many more plants are dead than same time last year. As you note, it's the second severe winter in a row.

I'll get some photos asap. My main question is whether to tear out all the struggling plants and start over, or fill in the spaces. Thank you, gardengal48.

I'll reserve judgment until I can see photos :-) And I'd like see exactly what this "hill" is like afa size and slope are concerned.

Are you willing to go to the time and expense involved in ripping it all out and starting over? Do you mulch? How much maintenance are you willing or interested in doing? And can you give us a sense of how much sun this hill receives on average?