I need help!

I have five Apline Spirea in the landscape. They were fine for a few years.
The last couple years they've leafed out fine with nice green leaves and
blossomed in late May/early June. Then the leaves turned yellow and eventually
dried up. The next spring the plant put out new shoots but the old stems were
dry and dead.

This year I wondered if they were even alive but they've put out nice new growth
(again old stems are dead) and are now starting to flower. But I notice the
leaves are already starting to go pale green or light yellow. I'm worried
they're going to die back again. The plants all face south. They are well
watered either by nature or me. They've been lightly fertilized.

Not sure whether I'm dealing with disease, insects, or something else. Also the
soil is high in clay. I have pygmy barberry, a shrub rose and some globe
arborvitae in the same area that are doing just fine. I lost a couple PJM
rhodedenron and hollies last year in this part of the landscape. I have lilacs,
a large spirea and several evergreen shrubs growing in similar soil elsewhere on
the property that are thriving. I'm in northern Illinois.

I'd appreciate any suggestions. I'm not much of a gardener but I do try to take
care of the landscape.